The finite element software is used to simulate the welding process of the body side wall of EMU at 400 km/h. The results show that when simulating the simultaneous welding by double manipulator arms,the postwelding deformation is the smallest under the condition of uniform distribution of special fixtures. The deformation presents a circular arc arched in reverse, and the both sides are symmetrical taking middle section as the center,and the maximum deformation is 5.13 mm. The actual welding test results coincide with the simulation results,the deformation trends are the same,and the deformation deviation is less than 0.5 mm. According to the deformation results,the predeformation is prefabricated and the postwelding deformation reduces further. The maximum postwelding deformation is 1.5 mm when the predeformation is 6 mm.