An automatic girth welding machine,which based on passive vision technology,was designed to solve the problems of low welding efficiency and poor welding quality of spherical tank girth welding. The walking tracks are attracted on the surface of the spherical tank through the switch magnetic seat,and the welding tractor walks along the tracks to complete the welding of the circular seam. The welding tracking system uses welding images,which collected by the passive vision sensor,to extract the deviation between the weld seam and the weld pool. Furthermore, the extracted deviation is used by the control system to control the rectifying mechanism and realize the welding seam tracking. The result shows that the tracking system corrects the welding deviation,which caused by the installation of walking track and greatly improves the precision of girth seam welding.