Structured P2P network is popular nowadays. However, the network needs reorganized when the peers are joining or leaving. To reduce the performance degradation caused by the reorganizations, we proposed a two-layer hybrid P2P network (HP2P). The upper layer is Chord network and the lower layer is cluster in HP2P. In this paper, the cluster is described in detail. Peers are organized into clusters, and the network reorganizations are restricted within the clusters where a peer can join or leave. Besides, each cluster is also enhanced through some specific mechanisms such as Supernode, metadata redundancy and gossip flooding. Consequently, the two layers P2P become more robust. Further, the high efficiency of Distributed Hash Tables(DHTs) can be retained, and the churn caused by excessive node joining and leaving can be reduced.