Mobile Peer to Peer (MP2P) networks provide decentralization, self-organization, scalability characters, but suffer from high latency and link break problems. In this paper, we study the cache/replication placement and cache update problems arising in such kind of networks. While researchers have proposed various replication placement algorithms to place data across the network to address the problem, it was proven as NP-hard. As a result, many heuristic algorithms have been brought forward for solving the problem. In our paper, we propose an effective and low cost cache placement strategy combined with an update scheme which can be easily implemented in a decentralized way. Extensive experiments are conducted to demonstrate the efficiency of the cache placement and update scheme.