Abstract: To meet the installation flatness and dimension requirements of urban rail transit vehicle aluminum alloy roof air-conditioning units, it is especially important to strictly match the air-conditioner mounting seat and air-conditioner frame dimensions, and control the flatness of the air-conditioner mounting seat. The structure of the air-conditioner mounting seat is designed and the production process is optimized. The positioning tool for the air-conditioner mounting seat is designed and manufactured, and the assembly welding process of the air-conditioner mounting seat and the air-conditioner frame on the roof is compiled. In view of the welding deformation of the roof, the process flow and roof structure are optimized and improved, and tests are conducted before and after the optimization. Results show that the air-conditioner mounting seat and air-conditioner frame produced according to the optimized process flow and roof structure can meet the installation flatness and dimension requirements of the air-conditioning unit, ensuring its successful installation.