It's a lot easier then I thought:
On the ssh server(which runs X-Client):
uncomment "#X11Forwarding no"
and modify it as "X11Forwarding yes"
On the ssh client(which has X-Server running)
uncomment "#ForwardX11 no"
and modify it as "ForwardX11 yes"
Or simply add "-X" argument when connecting to server.
If it success, after login to server, type
$ export | grep DISPLAY
you should see something like
declare -x DISPLAY="localhost:10.0"