diff --git a/install_trading.py b/install_trading.py
index 932df70..240c33a 100644
--- a/install_trading.py
+++ b/install_trading.py
@@ -13,6 +13,7 @@ from kline_stream import sse_format
 from position_sizing import (
     MODE_FIXED,
     MODE_RISK,
+    DEFAULT_MAX_ORDER_LOTS,
     calc_lots_by_risk,
     calc_order_tick_metrics,
     normalize_sizing_mode,
@@ -540,11 +541,19 @@ def install_trading(app, *, login_required, require_nav, get_db, get_setting, se
             if sl <= 0:
                 conn.close()
                 return jsonify({"ok": False, "error": "以损定仓模式须填写止损价"}), 400
-            lots_calc, err = calc_lots_by_risk(price, sl, direction, _capital(conn), get_risk_percent(get_setting), sym)
+            lots_calc, err = calc_lots_by_risk(
+                price, sl, direction, _capital(conn), get_risk_percent(get_setting), sym,
+            )
             if err:
                 conn.close()
                 return jsonify({"ok": False, "error": err}), 400
             lots = lots_calc or lots
+        if lots > DEFAULT_MAX_ORDER_LOTS:
+            conn.close()
+            return jsonify({
+                "ok": False,
+                "error": f"单笔手数 {lots} 超过上限 {DEFAULT_MAX_ORDER_LOTS}，请加大止损距离或改固定手数",
+            }), 400
         try:
             result = execute_order(
                 conn,
diff --git a/position_sizing.py b/position_sizing.py
index 42e9b0d..ab12e00 100644
--- a/position_sizing.py
+++ b/position_sizing.py
@@ -9,6 +9,9 @@ from contract_specs import get_contract_spec
 MODE_FIXED = "fixed"
 MODE_RISK = "risk"
 
+# 单笔报单手数上限（防止以损定仓在止损过近时算出超大手数）
+DEFAULT_MAX_ORDER_LOTS = 50
+
 
 def normalize_sizing_mode(raw: str) -> str:
     m = (raw or MODE_RISK).strip().lower()
@@ -63,8 +66,8 @@ def calc_lots_by_risk(
     if max_by_margin < 1:
         return None, "可用资金不足以覆盖 1 手保证金"
     lots = min(lots, max_by_margin)
-    if max_lots is not None:
-        lots = min(lots, int(max_lots))
+    cap_lots = max_lots if max_lots is not None else DEFAULT_MAX_ORDER_LOTS
+    lots = min(lots, cap_lots)
     return lots, None