In this paper, the collapse behaviors of thin plate with corrugated cross-section subjected to three-point bending are studied by using the finite element method. In order to estimate the energy-absorption characteristics of the beam, it is vital to understand the relation of load and displacement. It is found that the load decreases by flattening of the cross-section of the beam, and the flattening shape can be quantitatively expressed by using curvature radius of the plane of the top and bottom in the cross-section. Based on an idea that the external work is mainly expended by the flattening deformation of the cross-section, a new prediction method is proposed for estimating the relation of load and displacement. Its validity is verified by comparing with the numerical results by FEM under various conditions.