i just buy a new iphone 4S, i wanna sync my data from 3GS to new iphone but i forget the password for the 3GS

Connect your 3GS in recovery mode and restore, you'll get the option to reset the passcode during this process:

iOS: Unable to update or restore and iPhone and iPod touch: Wrong passcode results in red disabled screen

If you cannot remember the passcode, you will need to restore your device using the computer with which you last synced it. This allows you to reset your passcode and resync the data from the device (or restore from a backup). If you restore on a different computer that was never synced with the device, you will be able to unlock the device for use and remove the passcode, but your data will not be present. Refer to Updating and restoring iPhone, iPad and iPod touch software.