iMac WiFi connection to my router is unstable
The problem resolved after installing the newest Wi-Fi update.
http://www.macrumors.com/2012/02/24/apple-releases-imac-wi-fi-update-v1-0/

EDIT: after upgrading to Mountain Lion, the same issue occurs again. Anybody encounters the same problem?

Tried to install http://support.apple.com/kb/DL1501. But the package prompt out that 'The package require OS X 10.7' which means not be able to install on 10.8.