During electromagnetic(EM) rail launch armature, the moving loads due to armature sliding can cause rail dynamic response. In this paper, based on the basic theory of elastic beam, a model is developed to investigate the dynamic response. According to armature velocity and sliding distance which determined by current dates obtaining from launch test, rail deflection under moving loads is approached. The deformations of rail along with rail position and launch time are obtained, and also, deformation under rail repulsion and armature contact force are contrasted. These results can provide references for further design of supporting structure and further analysis of EM railgun life length.