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Free-riders and fake files are two important problems in P2P file sharing systems. Previous works have always used incentive mechanisms and trust mechanisms to address them respectively. In real systems however, a trust mechanism without incentive would face lack of users' enthusiasm and thus cause sparse relationship of direct trust while an incentive mechanism without trust could induce users' bad behavior. A novel reputation system is proposed in this paper that combines trust and incentive mechanisms. It uses files' vote and retention time, download volume and users' rank to construct a more extensive direct trust relationship and calculates a user's reputation with a multi-trust algorithm. It can identify fake files and provide service differentiation with reputation. Implementation and some security consideration in DHT are also discussed.