During the launching of electromagnetic railgun, movement of the armature will be influenced by many factors, such as electromagnetic force, armature initial positive pressure, friction force, air resistance and electrical erosion resistance when the armature is moving in the bore. The muzzle velocity of the armature will fluctuate in a certain range. To improve the precision of the muzzle velocity of the armature, based on the character of the uncertainty of friction and ablation degree of armature and rail, this paper presents a simulation model for the open loop control of the armature in the bore, considering the dynamic characteristics of the circuit model and armature. The relationship between the discharge time interval and muzzle velocity of the armature is obtained by simulation, the armature velocity closed loop control model is put forward, and the feasible scheme of armature velocity control is explored. The simulation results show that the closed-loop control can improve the control precision of the muzzle velocity of the armature.