I just formated my pc this now , and i installed my iTunes back . Then i try to sync some new MP3 to my iPod but it show a message said >

The iPod "Name" is synced with another iTunes library . Do you want to erase this iPod and sync with this iTunes library ?

An iPod can be synced with only one iTunes library at a time . Erasing and syncing replaces the contents of this iPod with the contents of this iTunes librady .

< What can i do to solve this problem , i don't want to erase because my iPod got a lot of Songs .

You didn't need to make a new thread for this issue. There are already iPod Support threads open.

Download Anapod, iPodRip, or similar software and extract all of the songs that are currently on your iPod. Then plug your iPod into iTunes, erase all the songs on the iPod. Put all of your newly extracted songs into your iTunes Library and sync with your iPod.