The stored-to-kinetic energy conversion efficiency of railgun system is investigated by simulation and experiment methods. There are many factors, which might affect the conversion efficiency of the railgun system. These factors include the parameter values of pulsed power supply (PPS), the velocity of armature, the inductance gradient of rail, and so on. To analyze the stored-to-kinetic energy conversion efficiency, a novel electric circuit simulation model for the railgun system is built. The simulation results show that the less resistance, the more inductance of PPS, the more inductance gradient of launcher, and the higher muzzle velocity of armature can improve the energy conversion efficiency of the railgun system. Some experimental results with the 6-MJ railgun system at Beijing Institute of Special Electromechanical Technology are also presented in this paper, which can illustrate the correctness of the analysis and the simulation results.