diff --git a/backend/app/services/concept_rotation_analyzer.py b/backend/app/services/concept_rotation_analyzer.py
index 0b574c4..b94964e 100644
--- a/backend/app/services/concept_rotation_analyzer.py
+++ b/backend/app/services/concept_rotation_analyzer.py
@@ -206,11 +206,24 @@ def _compute_rotation_signals(dates: list[str], columns: dict) -> dict:
 # ================================================================
 
 def _fmt_pct(v) -> str:
+    """概念/行业涨幅: 小数口径 (0.0522 = +5.22%), 展示前乘 100。"""
     if v is None:
         return "—"
     return f"{v*100:+.2f}%"
 
 
+def _fmt_index_pct(v) -> str:
+    """指数涨跌幅: 百分数口径 (CONTRIBUTING §3.1), 直接展示, 不能再乘一次 100。
+
+    build_market_overview 的 indices[].change_pct 在数据边界已转成百分数
+    (quote_service._build_index_quotes 与 _index_quotes 的 DB 兜底都已乘过 100),
+    与 market_recap._build_indices_block 的展示口径一致。
+    """
+    if v is None:
+        return "—"
+    return f"{v:+.2f}%"
+
+
 def _build_market_block(overview: dict) -> str:
     """大盘背景精简块 (复用 market_overview 已算好的字段)。"""
     indices = overview.get("indices") or []
@@ -222,7 +235,7 @@ def _build_market_block(overview: dict) -> str:
     for idx in indices[:4]:
         name = idx.get("name") or idx.get("symbol") or "?"
         chg = idx.get("change_pct")
-        idx_lines.append(f"{name} {_fmt_pct(chg)}")
+        idx_lines.append(f"{name} {_fmt_index_pct(chg)}")
     idx_str = " / ".join(idx_lines) or "指数缺失"
 
     total_amount = (amt.get("total") or 0) / 1e8  # 元 → 亿
diff --git a/backend/tests/test_rotation_prompt_index_pct.py b/backend/tests/test_rotation_prompt_index_pct.py
new file mode 100644
index 0000000..946a25b
--- /dev/null
+++ b/backend/tests/test_rotation_prompt_index_pct.py
@@ -0,0 +1,56 @@
+"""AI 轮动分析提示词里的「大盘背景」指数涨跌幅必须按百分数口径。
+
+CONTRIBUTING §3.1: build_market_overview 的 indices[].change_pct 是百分数
+(quote_service._build_index_quotes 在数据边界已乘过 100, DB 兜底路径同样乘过 100)。
+概念/行业涨幅则是小数。两者共用一个 _fmt_pct 会把指数再放大 100 倍,
+喂给 LLM 的大盘背景变成「上证指数 +123.00%」。
+"""
+from __future__ import annotations
+
+from app.services.concept_rotation_analyzer import (
+    _build_market_block,
+    _build_signal_block,
+)
+
+
+def _overview() -> dict:
+    """与 build_market_overview 返回结构一致的最小切片。"""
+    return {
+        "indices": [
+            {"symbol": "000001.SH", "name": "上证指数", "change_pct": 1.23},
+            {"symbol": "399001.SZ", "name": "深证成指", "change_pct": -0.45},
+        ],
+        "emotion": {"score": 62, "label": "偏暖"},
+        "limit": {"limit_up": 68, "broken": 20, "limit_down": 3, "max_boards": 5},
+        "amount": {"total": 1.2e12},
+    }
+
+
+def test_index_change_pct_is_not_scaled_again():
+    block = _build_market_block(_overview())
+
+    assert "上证指数 +1.23%" in block
+    assert "深证成指 -0.45%" in block
+    assert "123.00%" not in block
+
+
+def test_missing_index_change_pct_renders_placeholder():
+    overview = _overview()
+    overview["indices"][0]["change_pct"] = None
+
+    assert "上证指数 —" in _build_market_block(overview)
+
+
+def test_dimension_pct_is_still_scaled_from_decimal():
+    """概念/行业涨幅仍是小数口径, 必须乘 100 后展示 (不能被一起改掉)。"""
+    items = [{
+        "concept": "人工智能",
+        "ranks": [3, 1],
+        "pcts": [0.05, 0.07],
+        "avg_rank": 2.0,
+        "rank_std": 1.0,
+    }]
+
+    block = _build_signal_block("主线", items)
+
+    assert "区间均涨 +6.00%" in block