In Tohoku earthquake (March 11, 2011), Tsunami flotsams attacked buildings and then caused terrible damage to many of them. For the purpose of preventing the damage, the authors have experimentally studied the loading capacity of concrete filled steel tubular (CFT) members subjected to lateral impacts, so that the CFT members are expected to have effective resistance to the impacts like the collision of flotsams. The purpose of this paper is to derive an evaluation method for the capacity of CFT members against lateral impacts by theoretical manner. The impact response analysis is performed by using a frame analysis program. From the numerical parametric study by using the analytical model, the lateral impact response of CFT member under the constant axial load has been clarified. Being based on the results and theoretical consideration, it is proposed an evaluation method of the allowable input energy by the lateral impact for a CFT member. The method takes both of the energy absorbing capacity of a CFT member and the potential energy release by the axis load into consideration.