In unstructured peer-to-peer (P2P) systems, the mechanism of a peer randomly joining and leaving a P2P network causes topology mismatching between the P2P logical overlay network and the physical underlying network, causing a large volume of redundant traffic in the Internet. In order to alleviate the mismatching problem, we propose Adaptive Connection Establishment (ACE), an algorithm of building an overlay multicast tree among each source node and the peers within a certain diameter from the source peer, and further optimizing the neighbor connections that are not on the tree, while retaining the search scope. Our simulation study shows that this approach can effectively solve the mismatching problem and significantly reduce P2P traffic. We further study the tradeoffs between the topology optimization rate and the information exchange overhead by changing the diameter used to build the tree.