diff --git a/tools/harness-thci/OpenThread.py b/tools/harness-thci/OpenThread.py
index 99a4f5e6e..0c785c83e 100644
--- a/tools/harness-thci/OpenThread.py
+++ b/tools/harness-thci/OpenThread.py
@@ -500,7 +500,7 @@ class OpenThread(IThci):
         finalMac = ':'.join(a + b + c + d for a,b,c,d in zip(hexIter, hexIter,hexIter,hexIter))
         prefix = str(finalMac)
         strIp6Prefix = prefix[:20]
-        return strIp6Prefix +':'
+        return strIp6Prefix +'::'
 
     def __convertLongToString(self, iValue):
         """convert a long hex integer to string