diff --git a/backend/app/domain/entities/research.py b/backend/app/domain/entities/research.py
index 21e48a3..62f06f5 100644
--- a/backend/app/domain/entities/research.py
+++ b/backend/app/domain/entities/research.py
@@ -45,6 +45,20 @@ class SelectionSpec(BaseModel):
     top_n: int = Field(default=30, ge=1, le=1000)
 
 
+class PortfolioSpec(BaseModel):
+    """组合构建（v2 §16）。MVP：等权；单股/行业上限等约束字段预留，
+    未建模约束在回测结果 unimplemented 中如实标注（禁止假装支持）。
+    """
+
+    weighting: str = Field(default="equal", pattern="^(equal)$")
+    max_position_pct: float | None = Field(
+        default=None, gt=0, le=1, description="单股最大权重（预留，未建模）"
+    )
+    max_industry_weight_pct: float | None = Field(
+        default=None, gt=0, le=1, description="行业最大权重（预留，未建模）"
+    )
+
+
 class CostSpec(BaseModel):
     """交易成本模型（单边比例）。
 
@@ -71,6 +85,7 @@ class ResearchSpec(BaseModel):
     rebalance: str = Field(default="monthly", pattern="^(weekly|monthly)$")
     period: tuple[date, date]
     costs: CostSpec = CostSpec()
+    portfolio: PortfolioSpec = PortfolioSpec()
     initial_capital: float = Field(default=1_000_000.0, gt=0)
 
     @field_validator("period")
diff --git a/backend/app/quant/local_engine.py b/backend/app/quant/local_engine.py
index f57ec98..58747cb 100644
--- a/backend/app/quant/local_engine.py
+++ b/backend/app/quant/local_engine.py
@@ -32,6 +32,7 @@ from app.quant.composite import (  # noqa: F401 —— re-export（模块化后
     cross_sectional_zscore,
 )
 from app.quant.evaluation import run_factor_test
+from app.quant.portfolio import equal_weight_budget, unimplemented_notes
 
 TRADING_DAYS = 252
 _DEFAULT_UNIMPLEMENTED = [
@@ -169,7 +170,7 @@ class TopKBacktestRunner:
             targets.append(s)
 
         if targets:
-            budget = cash / len(targets)
+            budget = equal_weight_budget(cash, len(targets))
             for s in targets:
                 c = float(close_d[s])
                 price_in = c * (1 + self.costs.slippage_rate)
@@ -263,7 +264,7 @@ class TopKBacktestRunner:
             positions=positions,
             trades=trades,
             turnover_pct=round(sum(notional) / max(init, 1) * 100, 2),
-            unimplemented=list(_DEFAULT_UNIMPLEMENTED),
+            unimplemented=list(_DEFAULT_UNIMPLEMENTED) + unimplemented_notes(self.spec.portfolio),
             config_snapshot=self.spec.model_dump(mode="json"),
         )
 
diff --git a/backend/app/quant/portfolio.py b/backend/app/quant/portfolio.py
new file mode 100644
index 0000000..d771d10
--- /dev/null
+++ b/backend/app/quant/portfolio.py
@@ -0,0 +1,31 @@
+"""Portfolio Engine（v2 §16）—— 组合构建模块（M8.2）。
+
+MVP：等权资金拆分（与既有 TopK 回测等权语义一致，行为收敛到本模块）；
+单股/行业上限等约束为预留字段，未建模时由回测器写入 unimplemented
+（禁止假装支持，AGENT.md §24）。
+"""
+
+from __future__ import annotations
+
+from app.domain.entities.research import PortfolioSpec
+
+
+def equal_weight_budget(cash: float, target_count: int) -> float:
+    """等权单标的预算：现金均分（target_count>0）。"""
+    if target_count <= 0:
+        return 0.0
+    return cash / target_count
+
+
+def unimplemented_notes(portfolio: PortfolioSpec) -> list[str]:
+    """组合层未建模项说明（默认空；设置约束即显式标注）。"""
+    notes: list[str] = []
+    if portfolio.max_position_pct is not None:
+        notes.append(
+            f"最大单股权重 {portfolio.max_position_pct:.0%} 约束未建模（Portfolio v1 仅等权）"
+        )
+    if portfolio.max_industry_weight_pct is not None:
+        notes.append(
+            f"最大行业权重 {portfolio.max_industry_weight_pct:.0%} 约束未建模（Portfolio v1 仅等权）"
+        )
+    return notes
diff --git a/backend/tests/test_selection_backtest_consistency.py b/backend/tests/test_selection_backtest_consistency.py
index 60d3ccf..a70e758 100644
--- a/backend/tests/test_selection_backtest_consistency.py
+++ b/backend/tests/test_selection_backtest_consistency.py
@@ -123,3 +123,26 @@ class TestSelectionBacktestConsistency:
             held, key=lambda s: res.candidates[[x.symbol for x in res.candidates].index(s)].score,
             reverse=True,
         )
+
+
+class TestPortfolioEngine:
+    def test_equal_weight_default_unchanged(self, daily_df) -> None:
+        """新增 PortfolioSpec 后默认配置回测结果与未设置前一致（回归由本文件首测已锁数值）。"""
+        from app.domain.entities.research import PortfolioSpec
+        from app.quant.engine import LocalEngine
+
+        spec = _spec(portfolio=PortfolioSpec())
+        result = LocalEngine().run_backtest(daily_df, spec)
+        assert result.summary.total_trades >= 0
+        # 未设约束 → 无组合约束说明
+        assert not any("约束未建模" in u for u in result.unimplemented)
+
+    def test_constraint_declared_in_unimplemented(self, daily_df) -> None:
+        from app.domain.entities.research import PortfolioSpec
+        from app.quant.engine import LocalEngine
+
+        spec = _spec(portfolio=PortfolioSpec(max_position_pct=0.1))
+        result = LocalEngine().run_backtest(daily_df, spec)
+        assert any("最大单股权重" in u for u in result.unimplemented)
+        # config_snapshot 记录组合配置
+        assert result.config_snapshot["portfolio"]["max_position_pct"] == 0.1