any one knows how to run some code during iphone startup,for example, within a framework of a plugin ?

Create a daemon, add a plist to /System/Library/LaunchDaemons
Details: launchd.plist(5) Mac OS X Manual Page

but i don't have the right to access directory /system under iPhone,how can i add a plist to it?

Huh? Did you forget to jailbreak?

if you have jailbroken make sure you have installed af2cadd to view root dirs, you're not gonna have much luck adding custom launchdaemons on a stock handset