Abstract: For welding of flat-roof in aluminum alloy car body of EMU, there are some defects such as incomplete fusion, cracks in the weld root and welding deformation in weld bead. So the causes of welding defects were studied in welding process of flat-roof in aluminum alloy car body of EMU, the welding parameters and control method of welding deformation were optimized. The results showed that the optimized can prevent welding defects such as incomplete fusion and root weld crack, and the overall flatness of the EMU aluminum alloy body flat can be controlled in less than 3 mm, the welding quality of body flat is effectively guaranteed.