Abstract: Based on large deformation dynamic equation and finite difference method, the elastic-plastic dynamic responses of a fully clamped shallow arch subjected to projectile impact were studied numerically. Through analyzing the instantaneous distribution of the internal forces during early time response, the elastic-plastic deformation mechanism and mode were illustrated. The deformation mode consists of six phases. In the early phase of impact, the propagation of plastic bending deformation from the impact point to the root of arch dominated the deformation mode. While in the later phase, the deformation of axial tension governed by the axial force was dominant. Under the high-velocity projectile impact, the uneven plastic bending deformation can cause the arch to produce reversed flexural deformation. It can also be seen that the dynamic response of arch was very sensitive to a certain range of impact speed. In this range, tiny increase of initial projectile velocity can cause rapid increase of displacement. However, variation of dynamic response with impact speed was continuous without abrupt lose of stability. Numerical results were in good agreement with the experimental results from literature [5].