Abstract: A quick design method of aluminum alloy extrusion train body cross section is proposed. According to the theoretical design of car body cross section, the horizontal and vertical displacements of car body, bogie, wheel rim, tread part, pantograph part and collector shoes are calculated, and the designable region of the vehicle maximum outer contour is obtained by using linear superposition method. On the basis of the vehicle outer contour, the internal contour is obtained by calculating the projection and curve fitting, then, the cavity is intersected and segmented, the cavity plate layout is designed. Finally, the TCL/TK language is used to finish the secondary development of the finite element pre-processing software Hypermesh and build up a program for quick design of the cavity of car body cross section. The value of the first order vertical bending frequency is used as the evaluation index for the feasibility of the body cross section design. The efficiency and the feasibility of this method is verified by practical application.