Abstract: Unstructured P2P is a kind of very popular network application, such as Gnutella, KaZaa, etc. But its open nature makes it easy to attack by malicious peers, such as uploading inauthentic files, spreading computer viruses, etc. Hence, to mitigate the adverse behavior of unreliable or malicious peers in unstructured P2P, a reputation model based on social rules is proposed in this paper. At the same time, the parameters of the reputation model and their computing methods are presented. Based on the reputation model, a trust mechanism for unstructured P2P is designed, and the computing method of reputation and satisfaction ratio in this trust mechanism is presented. A distributed storage mechanism of evaluation records of peers is designed, and a corresponding search mechanism of reputation information is presented. An algorithm of topology evolution based on trust is presented, and its simulation shows that through topology evolution the high reputation peers form the backbone of unstructured P2P, and that can effectively improve the system stability. Under static and dynamic network settings, the resistance experiments of the proposed trust mechanism are carried out, and the experiment results show that the proposed trust mechanism can effectively resist the attack of malicious peers.