i had downloaded the ios 4.0.2 ipsw from redmondpie website, and tried to restore my iphone 3gs(3.1.2, blackra1n unlocked) with the ipsw but it doesn't work, and an error comes up.
is it because apple had released ios 4.1 so it don't allow people to restore to 4.0.2, or it other problem?
Is there any solution to this?

Please search before posting. This is an extremely common (and exceedingly excruciatingly documented) issue on these forums.

Apple stopped signing iOS 4.0.2 over a month ago. Without an SHSH on file, you will only be able to restore to 4.1 (which contains a baseband that cannot currently be unlocked.)

Good luck!