This paper proposed a new constant-degree graph, DBR, which combined De Bruijn and Ring and kept 2 in-degrees and 2 out-degrees of a node. Adapting DBR to the dynamic network, we designed Tangram with constant-degree. Tangram was based on DHT. It was a scalable, completely decentralized and self-organizing structured P2P system. The routing algorithm of Tangram was composed of De Bruijn algorithm and Chord algorithm and achieved a time complexity of O(log N) per lookup request by using O(1) neighbors per node, where N was the network size. Experimental results show that Tangram is efficient.