Due to the anonymous participation and voluntary resource contributions in P2P system, the majority of peers lack eagemess to contribute resources or provide service to communities, which leads to the prevalence of “free riding”. Based on the analysis of “free riding”, according to Laffer Curve and U.S. Personal Income Tax, this paper gives a new incentive mechamsm “PIIM” from the personal income tax idea. Based on the idea that “Tax the rich more than the poor”, a second distribution of income is achieved, to be fair in P2P system. The use of campus network based on the experiments show that the incentive mechanism promotes social equality, and the goal that the node can participate in resources sharing is fulfilled.