In this paper, in order to establish a rational impact response analysis method of the RC rockfall protection galleries, elasto-plastic response analyses of a full-scale gallery with sand cushion under falling weight impact loading were conducted. Here, a new constitutive model for sand cushion was proposed, which was modified by adding the initial stiffness and yield plateau to a previous model. An applicability of the proposed model and analysis method were investigated comparing with the experimental results. From this study, it was seen that applying the proposed cushion model and analysis method, the dynamic response behavior of the gallery can be accurately predicted irrespective of the magnitude of the impact energy.