The sidewall of a high-speed train body is welded by multiple hollow extruded aluminum alloy sheets. Due to the material characteristics of aluminum alloy, excessive welding deformation is easy to occur in the welding process of the sidewall. The welding quality of the sidewall is mainly guaranteed by fixtures, and the reasonable arrangement of sidewall fixtures can reduce the welding deformation. In this paper, a robust design of sidewall fixtures is carried out based on the Taguchi method and the double response surface method. Firstly, simplified shell element models of the sidewall are established and verified, and a welding deformation evaluation index of the sidewall is put forward. Secondly, based on the inside and outside table experimental design, the Taguchi method is used to design the robust parameters of sidewall fixtures, and robust locating parameters of sidewall fixtures are determined according to the SNR. Thirdly, the mean and standard deviation models of welding deformation are established based on the double response surface method, and the robust design of sidewall fixtures are carried out with the minimization of the square sum of the mean close to the target and the standard deviation. Finally, by comparing the original scheme, the fixture design scheme based on the Taguchi method and the fixture design scheme based on the double response surface method, it shows that the robust scheme of sidewall fixtures based on the double response surface method is best.