diff --git a/auth/src/main/resources/db/auth-login-identifiers.sql b/auth/src/main/resources/db/auth-login-identifiers.sql
index 5f36ab421..f5161b038 100644
--- a/auth/src/main/resources/db/auth-login-identifiers.sql
+++ b/auth/src/main/resources/db/auth-login-identifiers.sql
@@ -20,15 +20,18 @@ DROP PROCEDURE IF EXISTS auth_backfill_login_phone;
 DELIMITER $$
 CREATE PROCEDURE auth_backfill_login_phone()
 BEGIN
-  IF EXISTS (
-      SELECT user.phone
-        FROM baiyue_iam.iam_user user
-       WHERE user.deleted = 0 AND user.phone REGEXP '^1[3-9][0-9]{9}$'
-       GROUP BY user.phone HAVING COUNT(DISTINCT user.auth_user_id) > 1) THEN
-    SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'IAM 存在重复手机号，禁止建立 Auth 唯一登录索引';
-  END IF;
+  UPDATE auth_account
+     SET phone = NULL
+   WHERE phone IS NOT NULL AND phone NOT REGEXP '^1[3-9][0-9]{9}$';
   UPDATE auth_account account
   JOIN baiyue_iam.iam_user user ON user.auth_user_id = account.id AND user.deleted = 0
+  JOIN (
+    SELECT phone
+      FROM baiyue_iam.iam_user
+     WHERE deleted = 0 AND phone REGEXP '^1[3-9][0-9]{9}$'
+     GROUP BY phone
+    HAVING COUNT(DISTINCT auth_user_id) = 1
+  ) unique_phone ON unique_phone.phone = user.phone
      SET account.phone = user.phone
    WHERE account.phone IS NULL AND user.phone REGEXP '^1[3-9][0-9]{9}$';
   IF NOT EXISTS (
diff --git a/auth/src/test/java/com/baiyue/auth/repository/AuthLoginIdentifiersSqlContractTest.java b/auth/src/test/java/com/baiyue/auth/repository/AuthLoginIdentifiersSqlContractTest.java
new file mode 100644
index 000000000..7a9e163ab
--- /dev/null
+++ b/auth/src/test/java/com/baiyue/auth/repository/AuthLoginIdentifiersSqlContractTest.java
@@ -0,0 +1,32 @@
+package com.baiyue.auth.repository;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.core.io.ClassPathResource;
+
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class AuthLoginIdentifiersSqlContractTest {
+    @Test
+    void backfillsOnlyUniquelyOwnedPhonesAndKeepsDuplicateLegacyPhonesUnbound() throws Exception {
+        assertSafeBackfill(new ClassPathResource("db/auth-login-identifiers.sql")
+            .getContentAsString(StandardCharsets.UTF_8));
+        assertSafeBackfill(Files.readString(unifiedSql(), StandardCharsets.UTF_8));
+    }
+
+    private static void assertSafeBackfill(String sql) {
+        assertThat(sql)
+            .contains("phone NOT REGEXP '^1[3-9][0-9]{9}$'")
+            .contains("HAVING COUNT(DISTINCT auth_user_id) = 1")
+            .contains("ADD UNIQUE KEY uk_auth_account_phone (phone)")
+            .doesNotContain("IAM 存在重复手机号，禁止建立 Auth 唯一登录索引");
+    }
+
+    private static Path unifiedSql() {
+        Path path = Path.of("deploy", "sql", "头程数据库统一增量-MySQL8.sql");
+        return Files.exists(path) ? path : Path.of("..", "deploy", "sql", "头程数据库统一增量-MySQL8.sql");
+    }
+}
diff --git a/deploy/sql/头程数据库统一增量-MySQL8.sql b/deploy/sql/头程数据库统一增量-MySQL8.sql
index 3b426487e..2cdd21695 100644
--- a/deploy/sql/头程数据库统一增量-MySQL8.sql
+++ b/deploy/sql/头程数据库统一增量-MySQL8.sql
@@ -17077,6 +17077,52 @@ CREATE TABLE IF NOT EXISTS fl_dropship_pallet_carton (
   CONSTRAINT fk_dropship_carton_label FOREIGN KEY (carton_label_id) REFERENCES fl_inbound_carton_label(id)
 ) ENGINE=InnoDB COMMENT='一件代发托盘内逐箱SKU和入仓单明细';
 
+-- Auth 登录手机号只从 IAM 中唯一归属的中国大陆手机号回填；存量重复号保持 NULL。
+USE baiyue_auth;
+DROP PROCEDURE IF EXISTS auth_add_login_phone;
+DELIMITER $$
+CREATE PROCEDURE auth_add_login_phone()
+BEGIN
+  IF NOT EXISTS (
+      SELECT 1 FROM information_schema.columns
+       WHERE table_schema = 'baiyue_auth' AND table_name = 'auth_account' AND column_name = 'phone') THEN
+    ALTER TABLE auth_account ADD COLUMN phone VARCHAR(32) NULL COMMENT '全局唯一中国大陆登录手机号' AFTER username;
+  END IF;
+END$$
+DELIMITER ;
+CALL auth_add_login_phone();
+DROP PROCEDURE auth_add_login_phone;
+
+DROP PROCEDURE IF EXISTS auth_backfill_login_phone;
+DELIMITER $$
+CREATE PROCEDURE auth_backfill_login_phone()
+BEGIN
+  UPDATE auth_account
+     SET phone = NULL
+   WHERE phone IS NOT NULL AND phone NOT REGEXP '^1[3-9][0-9]{9}$';
+  UPDATE auth_account account
+  JOIN baiyue_iam.iam_user user ON user.auth_user_id = account.id AND user.deleted = 0
+  JOIN (
+    SELECT phone
+      FROM baiyue_iam.iam_user
+     WHERE deleted = 0 AND phone REGEXP '^1[3-9][0-9]{9}$'
+     GROUP BY phone
+    HAVING COUNT(DISTINCT auth_user_id) = 1
+  ) unique_phone ON unique_phone.phone = user.phone
+     SET account.phone = user.phone
+   WHERE account.phone IS NULL AND user.phone REGEXP '^1[3-9][0-9]{9}$';
+  IF NOT EXISTS (
+      SELECT 1 FROM information_schema.statistics
+       WHERE table_schema = 'baiyue_auth' AND table_name = 'auth_account'
+         AND index_name = 'uk_auth_account_phone') THEN
+    ALTER TABLE auth_account ADD UNIQUE KEY uk_auth_account_phone (phone);
+  END IF;
+END$$
+DELIMITER ;
+CALL auth_backfill_login_phone();
+DROP PROCEDURE auth_backfill_login_phone;
+
+USE baiyue_firstleg;
 CALL baiyue_firstleg.assert_unified_final_schema();
 DROP PROCEDURE IF EXISTS baiyue_firstleg.assert_unified_final_schema;
 SELECT 'PASS' AS migration_status,