diff --git a/backend/app/services/payroll_calculation.py b/backend/app/services/payroll_calculation.py
index 96e3ba8..e46f398 100644
--- a/backend/app/services/payroll_calculation.py
+++ b/backend/app/services/payroll_calculation.py
@@ -25,7 +25,7 @@ from app.services import audit_service, formula, payroll_months
 from app.services.audit_service import AuditRecord
 from app.services.param_service import resolve_snapshot
 from app.services.tplus.calculation_preview import _parameters
-from app.services.tplus.fifo import _beneficiary, amount_text, money, preview_group
+from app.services.tplus.fifo import START, _beneficiary, amount_text, money, preview_group
 from app.services.tplus.workbench import digest
 
 
@@ -308,11 +308,13 @@ def _calculate_month(session, business, record, month, reason=None):
             }
     if set(previous) - current.keys():
         raise BadRequestError("历史已核算事件发生变化，请核对更正或调整，不可直接覆盖")
+    # 启用前的发货由期初应收承接，不再进入新业务的起付比例分母。
     sales = sum(
         (
             money(s["amount"])
             for s in values["shipments"]
-            if s["date"] <= until.isoformat() and not s.get("performance_excluded")
+            if START.isoformat() <= s["date"] <= until.isoformat()
+            and not s.get("performance_excluded")
         ),
         Decimal(0),
     )
diff --git a/backend/tests/unit/test_formal_payroll.py b/backend/tests/unit/test_formal_payroll.py
index 0531c1f..01f65ab 100644
--- a/backend/tests/unit/test_formal_payroll.py
+++ b/backend/tests/unit/test_formal_payroll.py
@@ -22,6 +22,7 @@ from app.models import (
     Personnel,
     Product,
     Receipt,
+    Shipment,
     ShipmentItem,
     TPlusSourceLink,
     TPlusWorkRun,
@@ -268,6 +269,61 @@ def test_起付百分之六十与平方权重(payroll_case, amount, performance,
     )
 
 
+@pytest.mark.parametrize("legacy_date", [date(2026, 2, 10), date(2026, 2, 28)])
+def test_历史发货不重复计入新业务起付分母(payroll_case, legacy_date):
+    """旧应收先冲抵，启用前发货不得再次压低新业务的回款比例。"""
+    s, user, _ = payroll_case
+    business = imported(payroll_case)
+    record = payroll_calculation.latest_input(s, business.id)
+    data = copy.deepcopy(record.data_json)
+    legacy = Shipment(
+        business_id=business.id,
+        shipment_no="SH-LEGACY",
+        ship_date=legacy_date,
+        total_amount=Decimal("100"),
+        freight_amount=Decimal("0"),
+    )
+    s.add(legacy)
+    s.flush()
+    original = s.get(ShipmentItem, data["maps"]["shipment_items"][data["shipments"][0]["line_id"]])
+    item = ShipmentItem(
+        shipment_id=legacy.id,
+        product_id=original.product_id,
+        product_name=original.product_name,
+        spec_model=original.spec_model,
+        unit=original.unit,
+        quantity=Decimal("2"),
+        sale_price=Decimal("50"),
+        amount=Decimal("100"),
+    )
+    s.add(item)
+    s.flush()
+    data["shipments"].append(
+        {
+            **data["shipments"][0],
+            "source_id": "99",
+            "line_id": "99",
+            "date": legacy_date.isoformat(),
+        }
+    )
+    data["maps"]["shipments"]["99"] = legacy.id
+    data["maps"]["shipment_items"]["99"] = item.id
+    data["group"]["opening_balance"] = "100"
+    data["receipts"][0]["amount"] = "200"
+    s.get(Receipt, data["maps"]["receipts"][data["receipts"][0]["source_id"]]).amount = Decimal(
+        "200"
+    )
+    replace_input(s, record, data)
+
+    result = payroll_calculation.calculate(s, business.id, "2026-03", operator=user)
+
+    assert result["start_payment_ok"] is True
+    assert result["performance"] == result["payable"] == "0.20"
+    allocations = list(s.scalars(select(PayrollAllocation)))
+    assert sum(row.amount for row in allocations) == Decimal("100.00")
+    assert all(row.data_json["shipment_date"] == "2026-03-10" for row in allocations)
+
+
 def test_跨月续算仅计应付差额且离职未发给接手人(payroll_case):
     s, user, _manager = payroll_case
     business = imported(payroll_case)