diff --git a/source/libs/scalar/src/sclfunc.c b/source/libs/scalar/src/sclfunc.c
index b9af716929..1b7559a207 100644
--- a/source/libs/scalar/src/sclfunc.c
+++ b/source/libs/scalar/src/sclfunc.c
@@ -654,9 +654,12 @@ int32_t substrFunction(SScalarParam *pInput, int32_t inputNum, SScalarParam *pOu
   SColumnInfoData *pInputData = pInput->columnData;
   SColumnInfoData *pOutputData = pOutput->columnData;
 
-  int32_t outputLen = pInputData->varmeta.length * pInput->numOfRows;
-  char   *outputBuf = taosMemoryCalloc(outputLen, 1);
-  char   *output = outputBuf;
+  int32_t outputLen = pInputData->info.bytes;
+  char *outputBuf = taosMemoryMalloc(outputLen);
+  if (outputBuf == NULL) {
+    qError("substr function memory allocation failure. size: %d", outputLen);
+    return TSDB_CODE_OUT_OF_MEMORY;
+  }
 
   for (int32_t i = 0; i < pInput->numOfRows; ++i) {
     if (colDataIsNull_s(pInputData, i)) {
@@ -676,14 +679,16 @@ int32_t substrFunction(SScalarParam *pInput, int32_t inputNum, SScalarParam *pOu
       startPosBytes = TMAX(startPosBytes, 0);
     }
 
+    char   *output = outputBuf;
     int32_t resLen = TMIN(subLen, len - startPosBytes);
     if (resLen > 0) {
       memcpy(varDataVal(output), varDataVal(input) + startPosBytes, resLen);
+      varDataSetLen(output, resLen);
+    } else {
+      varDataSetLen(output, 0);
     }
 
-    varDataSetLen(output, resLen);
     colDataSetVal(pOutputData, i, output, false);
-    output += varDataTLen(output);
   }
 
   pOutput->numOfRows = pInput->numOfRows;