diff --git a/fe/fe-core/src/main/java/org/apache/doris/nereids/trees/plans/logical/LogicalSetOperation.java b/fe/fe-core/src/main/java/org/apache/doris/nereids/trees/plans/logical/LogicalSetOperation.java
index 9a5900908c..dd392ad16b 100644
--- a/fe/fe-core/src/main/java/org/apache/doris/nereids/trees/plans/logical/LogicalSetOperation.java
+++ b/fe/fe-core/src/main/java/org/apache/doris/nereids/trees/plans/logical/LogicalSetOperation.java
@@ -144,8 +144,8 @@ public abstract class LogicalSetOperation extends AbstractLogicalPlan implements
             Slot left = child(0).getOutput().get(i);
             Slot right = child(1).getOutput().get(i);
             DataType compatibleType = getAssignmentCompatibleType(left.getDataType(), right.getDataType());
-            Expression newLeft = TypeCoercionUtils.castIfNotSameType(left, compatibleType);
-            Expression newRight = TypeCoercionUtils.castIfNotSameType(right, compatibleType);
+            Expression newLeft = TypeCoercionUtils.castIfNotSameTypeStrict(left, compatibleType);
+            Expression newRight = TypeCoercionUtils.castIfNotSameTypeStrict(right, compatibleType);
             if (newLeft instanceof Cast) {
                 newLeft = new Alias(newLeft, left.getName());
             }
diff --git a/fe/fe-core/src/main/java/org/apache/doris/nereids/util/TypeCoercionUtils.java b/fe/fe-core/src/main/java/org/apache/doris/nereids/util/TypeCoercionUtils.java
index 2a6ba2f2ee..5991137f11 100644
--- a/fe/fe-core/src/main/java/org/apache/doris/nereids/util/TypeCoercionUtils.java
+++ b/fe/fe-core/src/main/java/org/apache/doris/nereids/util/TypeCoercionUtils.java
@@ -423,6 +423,20 @@ public class TypeCoercionUtils {
         }
     }
 
+    /**
+     * like castIfNotSameType does, but varchar or char type would be cast to target length exactly
+     */
+    public static Expression castIfNotSameTypeStrict(Expression input, DataType targetType) {
+        if (input.isNullLiteral()) {
+            return new NullLiteral(targetType);
+        } else if (input.getDataType().equals(targetType) || isSubqueryAndDataTypeIsBitmap(input)) {
+            return input;
+        } else {
+            checkCanCastTo(input.getDataType(), targetType);
+            return unSafeCast(input, targetType);
+        }
+    }
+
     private static boolean isSubqueryAndDataTypeIsBitmap(Expression input) {
         return input instanceof SubqueryExpr && input.getDataType().isBitmapType();
     }
diff --git a/regression-test/suites/nereids_p0/union/test_union.groovy b/regression-test/suites/nereids_p0/union/test_union.groovy
index 536e52ba44..fda90bc1ca 100644
--- a/regression-test/suites/nereids_p0/union/test_union.groovy
+++ b/regression-test/suites/nereids_p0/union/test_union.groovy
@@ -320,4 +320,27 @@ suite("test_union") {
       sql """ insert into  ${tblName1} values("1", "2", "3"),("2", "3", "4") """
 
       qt_sql """ select a_key from (select * from ${tblName1} UNION ALL select * from ${tblName2}) t ORDER BY a_key + 1"""
+
+      sql """DROP TABLE IF EXISTS c5770_t1"""
+      sql """CREATE TABLE c5770_t1 (
+            `id` varchar(10) NULL
+            ) ENGINE=OLAP
+            UNIQUE KEY(`id`)
+            DISTRIBUTED BY HASH(`id`) BUCKETS AUTO
+            PROPERTIES (
+            "replication_allocation" = "tag.location.default: 1"
+            );"""
+      sql """DROP TABLE IF EXISTS c5770_t2"""
+      sql """CREATE TABLE c5770_t2 (
+            `id` varchar(20) NULL
+            ) ENGINE=OLAP
+            UNIQUE KEY(`id`)
+            DISTRIBUTED BY HASH(`id`) BUCKETS AUTO
+            PROPERTIES (
+            "replication_allocation" = "tag.location.default: 1"
+            );"""
+      explain {
+        sql("""select id from ( select id from c5770_t1 union all select id from c5770_t2 ) t;""")
+        contains("CAST")
+      }
 }