To save more space for rootfs,I moved /usr/lib to /home/root/usr/lib,then I make a link to /usr/lib...but,when I reboot the device,it stop at nokia spash,sayig that"mal function device will shutdown in 10s"
It's obvious that I've been a fool...
what should I do? I really don't want to use flasher to flash the rootfs,because I didn't backup anything,I don't want to lost the sms record and contact and application etc.
It's there any way to read the rootfs in the occasion that my n900 can't boot up?Or I would like to flash a initfs or multiboot kernel that contains busybox to repair the system,where can I download it?
or you have any good idea? who can help me!?

I don't know if this will help you but search the wiki for "R&D mode". By activating that you may be able to boot you device and move the folder back to its right place.

I don't think R&D mode would actually help. Without backupmenu installed, or any other OS installed, it would be quite hard to try and backup the data or the setup you have made.

so, I just want to flash a multiboot kernel that contains busybox on it,where can i get it?

There is a method to install meego to an external SDcard and access the ubifs filesystem of maemo from there, at least that is what i remember reading some weeks ago. But i cannot give any more valuable information, sorry.