This paper studies how to suppress the abnormal vibration in a dynamic system wi th clearance. The abnormal vibration contains the bifurcations or chaotic motio n due to the change of some parameters of the nonlinear system. This paper prop oses a method to suppress chaotic motion by applying an external periodical for ce to the system. The periodical force is adjusted according to some real-time a lgorism related to an error between real-time states and the states when the sy stem is stable. This method has been applied to a two-degree-freedom impact vibration system. With numerical simulation, the effectiveness of the method is demonstrated.