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The problem of file pollution is prevalent in p2p file sharing system nowadays. The pollution reduces the availability of system sharply. Compared with peer reputation, object reputation is more efficient in face of pollution. Pollution proliferation model based on file object is presented. Vector space similarity is computed to weight the votes collected from other peers. Horting graph-theoretic approach is employed to deal with data sparsity problem. Self-adaptive reputation threshold scheme is adopted to estimate the file authenticity. An anti-pollution p2p file sharing system based on object reputation is established to improve the availability. Simulation results show that the system can distinguish between good and polluted files accurately and thwart the proliferation of pollution in the network with low communication cost. The system can subject freeriders to a penalty and do well in the presence of collusion and bad-mouthing attacks.