Q: the Photo Booth can not connect to the webcam！

Remove all connected USB devices.      Reboot, and try connecting that way.  If you are using a Mac that requires a keyboard and mouse that has USB, only use Apple's own keyboard and mouse through the built-in USB ports.  Without knowing more about what Mac model you have we can't give you specific advice.  

Please update your profile, as described below, or add the info in your post:

https://discussions.apple.com/docs/DOC-3602

http://support.apple.com/kb/HT2090

 

 

(Over time, Apple has changed the built-in camera's name on newer Macs from "iSight" to "FaceTime" and then to "FaceTime HD."  Regardless of the name of your Mac's built-in camera, the same info and troubleshooting applies.)