diff --git a/tools/harness-thci/OpenThread_WpanCtl.py b/tools/harness-thci/OpenThread_WpanCtl.py
index e9fd433b2..f3545160f 100644
--- a/tools/harness-thci/OpenThread_WpanCtl.py
+++ b/tools/harness-thci/OpenThread_WpanCtl.py
@@ -1595,7 +1595,7 @@ class OpenThread_WpanCtl(IThci):
             parameter = ''
 
             if P_slaac_preferred == 1:
-                parameter += ' -a'
+                parameter += ' -a -f'
 
             if P_stable == 1:
                 parameter += ' -s'