After a bit of search on google found a solution to my question.
First figure out the default wsl with the command:
C:\Windows\System32\wslconfig /list
Windows Subsystem for Linux Distributions:
Ubuntu-18.04
Legacy (Default)
then I set the default to Ubuntu-18.04 by
C:\Windows\System32\wslconfig /setdefault Ubuntu-18.04
Finally set the terminal path in Webstrom settings>tools>terminal to
C:\Windows\System32\wsl.exe
Now open the terminal it is correctly move the current directory to the /mnt/c/blah/blah