Hi Chris:
Thank you for your answer.
The external site is in another domain, but it's an specified web
application for authentication. When I login in the external site, it will
redirect to wiki with its user info.
I have seen an extension OpenID which I think works similiarly with my
requirement. OpenID will hijack wiki's login, and use its returned user as
an $wgUser. However, It seems quite complicated to replace wiki's internal
user login system. I have not fully understood the principle by now. And I
will try it with my best.
Best Regards!