Abstract: Automatic spot welding was an important way to improve production efficiency and manufacturing quality. How to plan robot welding trajectory was the key to achieve automatic welding. For the automatic spot welding station,based on the NX secondary development,combined with the simulation results of the SIEMENS industrial simulation software Tecnomatix platform,the automatic welding trajectory was analyzed.In accordance with the coordinate similarity transformation theory,the process of rotating transformation of the working coordinate system with respect to the absolute coordinate system was analyzed and the corresponding transformation matrix,namely the core algorithm of the program,was brought out. The C + + language platform was used to call the UFUN function for program development based on the NXOPEN API help documentation. The welding trajectory of the robot in the simulation,that is,the TCP pose,entered the NX in the form of batch creation of the coordinate system,and the 3 D data of the robot tool side welding gun was imported in batches corresponding to each coordinate system,and finally the simulation data was combined with the design data in the NX. Interference between the welding gun and other equipment on the design side was obtained. The result showed that the program had good practicability and reliability.