diff --git a/paimon-flink/paimon-flink-common/src/main/java/org/apache/paimon/flink/sink/FlinkSinkBuilder.java b/paimon-flink/paimon-flink-common/src/main/java/org/apache/paimon/flink/sink/FlinkSinkBuilder.java
index 5703c408243b..ecaa5678dd0b 100644
--- a/paimon-flink/paimon-flink-common/src/main/java/org/apache/paimon/flink/sink/FlinkSinkBuilder.java
+++ b/paimon-flink/paimon-flink-common/src/main/java/org/apache/paimon/flink/sink/FlinkSinkBuilder.java
@@ -265,6 +265,16 @@ protected DataStreamSink<?> buildDynamicBucketSink(
     }
 
     protected DataStreamSink<?> buildForFixedBucket(DataStream<InternalRow> input) {
+        int bucketNums = table.bucketSpec().getNumBuckets();
+        if (parallelism == null
+                && bucketNums < input.getParallelism()
+                && table.partitionKeys().isEmpty()) {
+            // For non-partitioned table, if the bucketNums is less than job parallelism.
+            LOG.warn(
+                    "For non-partitioned table, if bucketNums is less than the parallelism of inputOperator,"
+                            + " then the parallelism of writerOperator will be set to bucketNums.");
+            parallelism = bucketNums;
+        }
         DataStream<InternalRow> partitioned =
                 partition(
                         input,