There are two pivotal problems in unstructured P2P system:Topology self-organization and query algorithms,however,the former is more important because a well-organized topology can dramatically improve the performance of query algorithms.This paper develops a new mechanism to organize topology based on a hierarchical representation of interest—SACM(self adaptive community-based model).In this model,each node forms an interest which is dendriform from all resources it possesses.Then a bit sequence CID(community ID) is determined by the interest,and CID is the main metric to organize topology—nodes with the close CID will form a community which is a dense subgraph.SACM not only provides a mechanism to organize topology but also constructs the relationship between resources and topology which is the essential difference between structured and unstructured P2P.