Abstract: Unstructured Peer-to-Peer（P2P） systems are largely self-organized and scalable.However，the loose overlay structure and the mechanism of a peer randomly joining and leaving a P2P network cause topology mismatching between the P2P logical overlay network and the physical underlying network.Meanwhile，due to the inefficient overlay topology，the flooding-based search mechanisms cause a large volume of unnecessary traffic.Aiming at alleviating the mismatching problem and reducing the unnecessary traffic，the dynamic overlay topology optimization technique is proposed.The algorithm gets the current topology information through flooding queries among peers so as to alleviate the mismatching problem involving the optimization policy.