P2P worm exploits common vulnerabilities in Peer-to-Peer networks. It is a severe security threat. A P2P worm detection method was presented, which called P2P worm detection(PWD) based on the worm′s propagation characteristics. PWD consists of a preprocess procedure which is based on application identification technology and a P2P worm detection procedure which is based on unknown worm detection technology. Improved heuristics was also advanced to identify and disturbing traffic was eliminated as well as heuristics to detect P2P worm. The selection of detection period was discussed by applying methodologies of game theory. Simulation result and LAN-scope experimental result both indicate that PWD is an effective method to detect and block P2P worm.