In this paper, we present an anonymity scheme to achieve all the peers' anonymity in P2P reputation systems by changing pseudonym with the help of a trusted third party (TTP) server, such that neither the TTP server nor other peers can link each peer's pseudonyms. Compared with RuP, an existing representative scheme, our scheme has two main features. First, the TTP server's cost is reduced by more than half in terms of numbers of RSA decryption operations and message overhead, which makes our scheme be more scalable. Second, we also propose a reputation metric that can be integrated into P2P reputation systems that achieve peers' anonymity by changing pseudonym, to help estimate the trustworthiness of peers. Simulation results show the effectiveness of our proposed reputation metric.