Abstract: In order to accurately describe casing collapse under lateral load, we assume that the shape of the stress deformation zone of the casing is two arcs in plastic deformation. We establish casing damage mechanics balance relationship based on theprinciple of virtual work under lateral loads, then derive the casing damage mechanics model by the Fresnel integral. With the engineering practice of Daqing oilfield,weconduct casing collapse test. The new formula leads to a far smaller average relative error than that obtained from the API formula. From the viewpoint of energy, the paper provides a new research method for the study of casing collapse deformation under external load.