diff --git a/backend/app/api/expenses.py b/backend/app/api/expenses.py
index 4123a1df..db1570ed 100644
--- a/backend/app/api/expenses.py
+++ b/backend/app/api/expenses.py
@@ -290,7 +290,9 @@ def validate_reimbursement(db: Session, reimb: ExpenseReimbursement, rules: list
             })
         else:
             # 月度/年度累计: 统计同维度+同费用类型在周期内的已提交金额(含本次)
+            # 账套隔离 2026-09-21：累计口径限本账套（他账套在途不得污染本账套额度判定）
             q = db.query(func.coalesce(func.sum(ExpenseReimbursement.amount), 0)).filter(
+                ExpenseReimbursement.entity_id == reimb.entity_id,
                 ExpenseReimbursement.expense_type == reimb.expense_type,
                 ExpenseReimbursement.id != reimb.id,
                 ExpenseReimbursement.status.in_(["pending", "approved"]),
@@ -519,9 +521,13 @@ def resubmit_reimbursement(
     data: dict = None,
     db: Session = Depends(get_db),
     _=Depends(require_auth),
+    entity_id: int = Depends(get_entity_id),
 ):
-    """被驳回/打回后重新提交 → 重新自动校验"""
-    r = db.query(ExpenseReimbursement).filter(ExpenseReimbursement.id == reimb_id).first()
+    """被驳回/打回后重新提交 → 重新自动校验（账套隔离 2026-09-21：跨账套 → 404）"""
+    r = db.query(ExpenseReimbursement).filter(
+        ExpenseReimbursement.id == reimb_id,
+        ExpenseReimbursement.entity_id == entity_id,
+    ).first()
     if not r:
         raise HTTPException(404, "报销单不存在")
     if r.status not in ("returned", "rejected"):
diff --git a/backend/tests/test_cross_entity_isolation_expenses_residual_20260921.py b/backend/tests/test_cross_entity_isolation_expenses_residual_20260921.py
new file mode 100644
index 00000000..77d82c64
--- /dev/null
+++ b/backend/tests/test_cross_entity_isolation_expenses_residual_20260921.py
@@ -0,0 +1,225 @@
+"""CMA expenses 残余跨账套面收口回归测试 — 2026-09-21
+
+任务：fullstack-20260921-expenses-residual-entity-scope（batch5 派生单）
+
+本单范围（仅此两处，均在 backend/app/api/expenses.py）：
+  F1 (P1 跨租户写) resubmit_reimbursement 仅按 id 取行、无账套归属校验
+      → 本账套会话持他企报销单 id 即可改写金额并触发重新校验
+  F2 (P2 跨租户读污染判定) validate_reimbursement 月度/年度累计分支的累计查询未按
+      entity_id 过滤 → 他账套同费用类型在途金额计入本账套「月度/年度累计是否超限」判定
+
+每条均「跨账套负例 + 本账套正对照」。
+只跑内存 SQLite（tests/conftest.py 机制），零生产库 / 零生产 Redis 连接。
+"""
+from datetime import datetime
+
+from fastapi.testclient import TestClient
+from sqlalchemy.orm import Session
+
+from app.models import Entity, ExpenseReimbursement, ExpenseRule
+from tests.conftest import TEST_USER_PASSWORD, auth_header, create_test_user
+
+EXPENSES = "/api/cma/expenses"
+
+
+# ──────────────────────────────────────────────
+# 夹具
+# ──────────────────────────────────────────────
+
+def _ensure_entity(db: Session, eid: int, name: str) -> Entity:
+    ent = db.query(Entity).filter(Entity.id == eid).first()
+    if not ent:
+        ent = Entity(id=eid, name=name, short_name=name, status="active")
+        db.add(ent)
+        db.commit()
+        db.refresh(ent)
+    return ent
+
+
+def _mk_rule(db: Session, **kw) -> ExpenseRule:
+    defaults = {
+        "rule_name": "ZZEXP_RESIDUAL_RULE", "dimension": "expense_type",
+        "dimension_value": "", "expense_type": "office",
+        "limit_type": "monthly", "limit_amount": 1000.0,
+        "cycle": "monthly", "status": "active", "remark": "",
+    }
+    defaults.update(kw)
+    r = ExpenseRule(**defaults)
+    db.add(r)
+    db.commit()
+    db.refresh(r)
+    return r
+
+
+def _mk_reimb(db: Session, eid: int, no: str, amount: float = 100.0,
+              status: str = "pending", expense_type: str = "office",
+              expense_date: datetime = datetime(2026, 9, 10)) -> ExpenseReimbursement:
+    r = ExpenseReimbursement(entity_id=eid, reimb_no=no, applicant="张三",
+                             department="财务部", expense_type=expense_type,
+                             title=f"{no}-报销", amount=amount,
+                             expense_date=expense_date, status=status,
+                             check_result="pass")
+    db.add(r)
+    db.commit()
+    db.refresh(r)
+    return r
+
+
+def _login(client: TestClient, entity_id: int) -> str:
+    resp = client.post("/api/cma/auth/login", json={
+        "username": "testadmin",
+        "password": TEST_USER_PASSWORD,
+        "entity_id": entity_id,
+    })
+    assert resp.status_code == 200, f"登录 entity{entity_id} 失败: {resp.text[:300]}"
+    return resp.json()["token"]
+
+
+def _seed_two_entities(db: Session):
+    _ensure_entity(db, 1, "测试企业1")
+    _ensure_entity(db, 2, "测试企业2")
+    create_test_user(db)
+
+
+def _check_entry(detail, rule_type: str) -> dict:
+    for c in (detail or []):
+        if c.get("rule_type") == rule_type:
+            return c
+    raise AssertionError(f"check_detail 缺少 {rule_type} 条目: {detail}")
+
+
+# ──────────────────────────────────────────────
+# F1. resubmit_reimbursement 账套归属（跨账套 → 404）
+# ──────────────────────────────────────────────
+
+def test_resubmit_cross_entity_404(client: TestClient, db: Session):
+    """e1 会话持 e2 报销单 id 重新提交（含改金额）→ 404，且 e2 行零变更"""
+    _seed_two_entities(db)
+    other = _mk_reimb(db, 2, "ZZEXP_E2_RESUBMIT_1", amount=100.0, status="returned")
+    token1 = _login(client, 1)
+
+    r = client.post(f"{EXPENSES}/reimbursements/{other.id}/resubmit",
+                    headers=auth_header(token1), json={"amount": 9999.0})
+    assert r.status_code == 404, (
+        f"跨账套 resubmit 应 404，实际 {r.status_code} {r.text[:200]}")
+    assert "报销单不存在" in r.json()["detail"]
+
+    # 跨租户写必须零生效：金额/状态/校验结果均未改
+    db.expire_all()
+    row = db.query(ExpenseReimbursement).filter(
+        ExpenseReimbursement.id == other.id).first()
+    assert row is not None
+    assert row.amount == 100.0, f"跨账套 resubmit 改写了金额: {row.amount}"
+    assert row.status == "returned", f"跨账套 resubmit 改写了状态: {row.status}"
+
+
+def test_resubmit_own_entity_ok(client: TestClient, db: Session):
+    """本账套报销单重新提交仍可用 → 200（正对照，未被收口误杀）"""
+    _seed_two_entities(db)
+    mine = _mk_reimb(db, 1, "ZZEXP_E1_RESUBMIT_1", amount=100.0, status="returned")
+    token1 = _login(client, 1)
+
+    r = client.post(f"{EXPENSES}/reimbursements/{mine.id}/resubmit",
+                    headers=auth_header(token1), json={"amount": 80.0})
+    assert r.status_code == 200, r.text[:300]
+    assert r.json()["message"] == "已重新提交"
+    d = r.json()["data"]
+    assert d["amount"] == 80.0
+    assert d["status"] == "pending"
+
+    db.expire_all()
+    row = db.query(ExpenseReimbursement).filter(
+        ExpenseReimbursement.id == mine.id).first()
+    assert row.amount == 80.0 and row.status == "pending"
+
+
+# ──────────────────────────────────────────────
+# F2. validate_reimbursement 累计判定按账套（月度/年度）
+# ──────────────────────────────────────────────
+
+def test_monthly_cumulative_cross_entity_excluded(client: TestClient, db: Session):
+    """e2 在途金额不得计入 e1 月度累计：e1 单笔 200 / 月限额 1000 应通过"""
+    _seed_two_entities(db)
+    _mk_rule(db, rule_name="ZZEXP月限额1000", expense_type="office",
+             limit_type="monthly", limit_amount=1000.0, cycle="monthly")
+    _mk_reimb(db, 2, "ZZEXP_E2_MONTH_900", amount=900.0, status="pending",
+              expense_date=datetime(2026, 9, 5))
+    token1 = _login(client, 1)
+
+    r = client.post(EXPENSES + "/reimbursements", headers=auth_header(token1),
+                    json={"expense_type": "office", "title": "e1月度单",
+                          "amount": 200.0, "expense_date": "2026-09-15"})
+    assert r.status_code == 200, r.text[:300]
+    d = r.json()["data"]
+    assert d["auto_check_passed"] is True, (
+        f"他账套在途金额污染了本账套月度判定: {d['check_detail']}")
+    assert d["status"] == "pending"
+
+    entry = _check_entry(d["check_detail"], "月度累计")
+    assert entry["used"] == 0.0, f"月度累计 used 含他账套金额: {entry}"
+    assert entry["actual"] == 200.0
+    assert entry["passed"] is True
+
+
+def test_monthly_cumulative_own_entity_still_counted(client: TestClient, db: Session):
+    """本账套月度累计仍照常计入：e1 已有 500，再报 600 → 超 1000 自动打回"""
+    _seed_two_entities(db)
+    _mk_rule(db, rule_name="ZZEXP月限额1000b", expense_type="office",
+             limit_type="monthly", limit_amount=1000.0, cycle="monthly")
+    _mk_reimb(db, 1, "ZZEXP_E1_MONTH_500", amount=500.0, status="pending",
+              expense_date=datetime(2026, 9, 5))
+    token1 = _login(client, 1)
+
+    r = client.post(EXPENSES + "/reimbursements", headers=auth_header(token1),
+                    json={"expense_type": "office", "title": "e1月度超限",
+                          "amount": 600.0, "expense_date": "2026-09-16"})
+    assert r.status_code == 200, r.text[:300]
+    d = r.json()["data"]
+    assert d["auto_check_passed"] is False, (
+        f"本账套累计未生效（收口过度）: {d['check_detail']}")
+    assert d["status"] == "returned"
+    entry = _check_entry(d["check_detail"], "月度累计")
+    assert entry["used"] == 500.0, f"本账套月度累计 used 应为 500: {entry}"
+    assert entry["actual"] == 1100.0
+
+
+def test_yearly_cumulative_cross_entity_excluded(client: TestClient, db: Session):
+    """e2 在途金额不得计入 e1 年度累计（跨月同年度）：e1 报 200 / 年限额 1000 应通过"""
+    _seed_two_entities(db)
+    _mk_rule(db, rule_name="ZZEXP年限额1000", expense_type="office",
+             limit_type="yearly", limit_amount=1000.0, cycle="yearly")
+    _mk_reimb(db, 2, "ZZEXP_E2_YEAR_900", amount=900.0, status="approved",
+              expense_date=datetime(2026, 3, 5))
+    token1 = _login(client, 1)
+
+    r = client.post(EXPENSES + "/reimbursements", headers=auth_header(token1),
+                    json={"expense_type": "office", "title": "e1年度单",
+                          "amount": 200.0, "expense_date": "2026-09-15"})
+    assert r.status_code == 200, r.text[:300]
+    d = r.json()["data"]
+    assert d["auto_check_passed"] is True, (
+        f"他账套在途金额污染了本账套年度判定: {d['check_detail']}")
+    entry = _check_entry(d["check_detail"], "年度累计")
+    assert entry["used"] == 0.0, f"年度累计 used 含他账套金额: {entry}"
+    assert entry["actual"] == 200.0
+
+
+def test_yearly_cumulative_own_entity_still_counted(client: TestClient, db: Session):
+    """本账套年度累计仍照常计入：e1 已有 900（3月），再报 200 → 超 1000 自动打回"""
+    _seed_two_entities(db)
+    _mk_rule(db, rule_name="ZZEXP年限额1000b", expense_type="office",
+             limit_type="yearly", limit_amount=1000.0, cycle="yearly")
+    _mk_reimb(db, 1, "ZZEXP_E1_YEAR_900", amount=900.0, status="approved",
+              expense_date=datetime(2026, 3, 5))
+    token1 = _login(client, 1)
+
+    r = client.post(EXPENSES + "/reimbursements", headers=auth_header(token1),
+                    json={"expense_type": "office", "title": "e1年度超限",
+                          "amount": 200.0, "expense_date": "2026-09-16"})
+    assert r.status_code == 200, r.text[:300]
+    d = r.json()["data"]
+    assert d["auto_check_passed"] is False, (
+        f"本账套年度累计未生效（收口过度）: {d['check_detail']}")
+    entry = _check_entry(d["check_detail"], "年度累计")
+    assert entry["used"] == 900.0, f"本账套年度累计 used 应为 900: {entry}"
+    assert entry["actual"] == 1100.0