The Ultra-Dense Network (UDN) is a key technology in the future 5th Generation (5G) networks. UDN is a new network structure, which has a smaller cell radius. Deploying the Low Power Base Stations (LPBSs) is the core concept of UDN. Because of the increasing of cell density, the interference is more serious in UDN than in macro and micro heterogeneous scenario. It is known that clustering cooperation can decrease the interference. Firstly, the total uplink capacity expression is derived in the paper. Then a novel dynamic clustering algorithm is presented. In densely deployed small cell network, the algorithm can make a better tradeoff between the complexity and the system performance. And the inter-Mobile Station (MS) interference can be dereased. The significant capacity gain is shown in the simulation results and is better than these clustering algorithms which have been proposed.