imap protocol is enabled in my gmail account. and i beleive gmail support IMAP IDLE , at least https://blogs.oracle.com/chienr/entry/gmail_supports_imap_idle says that.
http://en.wikipedia.org/wiki/KMail#E-Mail says kmail support imap idle.
i configured my imap gmail account, enabled "启用服务器端订阅" which i guess means 'enable imap idle' in the advanced tab, and disabled periodically check email in the regular tab.

but aflter i have sent a mail to this account from another email server, kmail seems not receive this email instantly. i have to check email manually.
what's wrong?

let me answer this question myself.
imap idle is working now. i checked the wrong option. the underlined option in the following snapshot should be checked to make imap idle working.

IDLE works, but only for the INBOX folder at the moment.