The overlay network of unstructured P2P system is neither regular network,nor pure random network. The peers are usually not completely equivalent.They usually play different roles in the overlay network.This paper firstly investigates the relation among the degree distribution,the access frequency mode and the success rate, and then presents an optimal degree distribution model in terms of the popularity of data items.Finally,a feasible proactive replication is proposed to reach the expected degree distribution.The simulation shows that the proactive replication can improve the performance of the unstructured P2P.