The side wall unit is the skeleton of the vehicle body, which needs to be welded with the top cover and underframe respectively to support the whole vehicle body. The welding method of side wall unit is MIG welding, which is easy to produce welding deformation in the welding process;secondly, the welding process requirements of the side wall are relatively high and there are many welding processes. A series of processes such as upper door corner assembly welding, lower door corner assembly welding and side wall unit assembly welding need to be carried out respectively. Each process needs to be strictly controlled in order to ensure the overall dimen-sional accuracy. Through the analysis of the welding difficulties of the side wall unit, through the optimization of process parameters, the use of appropriate assembly tooling and reasonable welding sequence, the welding deformation of the side wall unit in the assembly welding process is effectively controlled, so that the process size and welding quality of the side wall unit are well guaranteed, which provides a reference for the subsequent assembly welding of similar side wall units.