Thanks, it works!
If I want to set root-squash, I can use -superuser none, root privilege will then be restricted. After I (root) mount, it is still owned by root, and 755, Which is fine.
but, it seems nobody then could create any files. what should I do to allow users to be able to rw? do I have to mount it with mode 777, only then others are able to rw?