diff --git a/cdc/sink/dmlsink/txn/mysql/mysql.go b/cdc/sink/dmlsink/txn/mysql/mysql.go
index 10e88ab792c..29897f88614 100644
--- a/cdc/sink/dmlsink/txn/mysql/mysql.go
+++ b/cdc/sink/dmlsink/txn/mysql/mysql.go
@@ -762,6 +762,24 @@ func (s *mysqlBackend) execDMLWithMaxRetries(pctx context.Context, dmls *prepare
 					start, s.changefeed, "BEGIN", dmls.rowCount, dmls.startTs)
 			}
 
+			// Set session variables first and then execute the transaction.
+			// we try to set write source for each txn,
+			// so we can use it to trace the data source
+			if err = s.setWriteSource(pctx, tx); err != nil {
+				err := logDMLTxnErr(
+					cerror.WrapError(cerror.ErrMySQLTxnError, err),
+					start, s.changefeed,
+					fmt.Sprintf("SET SESSION %s = %d", "tidb_cdc_write_source",
+						s.cfg.SourceID),
+					dmls.rowCount, dmls.startTs)
+				if rbErr := tx.Rollback(); rbErr != nil {
+					if errors.Cause(rbErr) != context.Canceled {
+						log.Warn("failed to rollback txn", zap.String("changefeed", s.changefeed), zap.Error(rbErr))
+					}
+				}
+				return 0, 0, err
+			}
+
 			// If interplated SQL size exceeds maxAllowedPacket, mysql driver will
 			// fall back to the sequantial way.
 			// error can be ErrPrepareMulti, ErrBadConn etc.
@@ -780,23 +798,6 @@ func (s *mysqlBackend) execDMLWithMaxRetries(pctx context.Context, dmls *prepare
 				}
 			}
 
-			// we try to set write source for each txn,
-			// so we can use it to trace the data source
-			if err = s.setWriteSource(pctx, tx); err != nil {
-				err := logDMLTxnErr(
-					cerror.WrapError(cerror.ErrMySQLTxnError, err),
-					start, s.changefeed,
-					fmt.Sprintf("SET SESSION %s = %d", "tidb_cdc_write_source",
-						s.cfg.SourceID),
-					dmls.rowCount, dmls.startTs)
-				if rbErr := tx.Rollback(); rbErr != nil {
-					if errors.Cause(rbErr) != context.Canceled {
-						log.Warn("failed to rollback txn", zap.String("changefeed", s.changefeed), zap.Error(rbErr))
-					}
-				}
-				return 0, 0, err
-			}
-
 			if err = tx.Commit(); err != nil {
 				return 0, 0, logDMLTxnErr(
 					cerror.WrapError(cerror.ErrMySQLTxnError, err),