Armature is the critical moving part of electromagnetic railgun,which is also the intermediary that converts electromagnetic energy of the railgun into kinetic energy of the projectile.The current and induced magnetic field in front of the armature are the fundamental reasons for the armature movement and directly determine the energy conversion rate of the railgun system.Based on uniform current hypothesis,the electromagnetic force model of the armature and the calculation model of electromagnetic induction intensity field in front of armature are established without considering the rail resistance and current skin effect.Based on the simulation results, the characteristics of magnetic induction intensity field are studied,and the effects of current length on armature magneto-dynamic force are analyzed.