Formation is an important issue in the multi-robot domain and is studied by many researchers.In complex environment,many robots,that is,a swarm of robots may be used to cooperate and to move in formation.In this situation,normal formation strategy may be difficult to gain good performance,for when robot number increase,the probability of some of the robots meeting obstacles or extraordinary situations which may cause them to maneuver from their formation positions increase greatly.In this paper,a hierarchy formation strategy of swarm robots is proposed.Formation architecture is divided into two layers: Formation layer and subgroup layer.When some of the robots need to maneuver,they can be divided into several subgroups and reform when possible.By using this method,the team gain better flexibility especially in dynamic unknown obstacle environment.Simulation is done to test the performance.