Abstract: This paper studies the lightweight method of the front rail of domestic mid-size cars. The average crushing force of the thin-walled beam composed of multiple right angles is derived based on the theory of the crushing of rectangular thin-walled beam. Meanwhile, the substructure dissipated energy and force are obtained by target cascading of the acceleration pulse in 65 km/h frontal crash. Based on the force and the lightweight target of substructures, the new cross-section of the rails is designed with the theory of crushing of multiple right angles thin-walled beam and bending of rectangular thin-walled beam. Then the wall thickness of the new rails is modified by finite element analysis. The weight of the new front rails designed by lightweight method is reduced by 20% and the frontal crashworthiness is remained reserved.