Aiming at the large amount of work based on traditional modeling method, a modular decomposition method for electromagnetic rail gun system is proposed. According to the rail gun characteristics, constructional trait and working mode, an applied simulation model is established, which consists of four typical components including power, rail, armature and projectile. It is convenient to analyze performance of rail gun with different power or armature by use of adjusting parameter or replacing module model. The simulation results show that increasing appropriate power supply voltage, designing proper structure of rail and the application of repairing technology of rail can improve the performance of rail gun. These analyses can provide new idea of model and simulation for electromagnetic rail gun in future work.