A method and a system for processing abnormally becoming power off of a multicast user terminal are disclosed, and the method includes: a multicast device deletes multicast group(s) corresponding to the multicast user terminal and not watched by any multicast user terminal, upon receipt of a notification from the multicast user terminal for exiting all multicast groups when the multicast user terminal becomes powered off and started up again. Embodiments of the present invention can save bandwidth of a line effectively, and enable bandwidths of other services to be guaranteed better.