In order to study the stability and bifurcation conditions of global and periodic motion for impact gear system with clearance, the single?degree?of?freedom nonlinear dynamic model of drive gear wheel is established. The non?smooth Melnikov′s theory is applied to study the global bifurcation condition of the heteroclinic orbits. The solutions of each piecewise differential equation are then solved, and each switch surface can be served as Poincaré section. Meanwhile, the method of the combined mapping is applied to analyze the periodic motion of the system. Finally, the motion states and bifurcation characteristics of the system are obtained under different parameters by numerical simulation, and the effectiveness of Melnikov method for analyzing non?smooth piecewise system can be verified.