Abstract: Plane autonomous mobile welding robot is used to work in cramped cabins,and the initial positioning of welding mainly relies on manpower. But sometimes it is hard for workers to enter a narrow lattice in a vertical space,and lifting the robot needs to overcome the gravity of the robot and the magnetic attraction force of the base. The operating characteristics of ultrasonic sensors was used,the robot was rotated to scan the wall to get the position information,and the least-square method was used to fit the time points when the robot is perpendicular to the wall during the whole scanning process. The mathematical model of robot margin adjustment was established,the monocular vision sensor was calibrated to correct the protection distance. The result shows that the system solves the positioning problem of the robot in the initial welding position,and it has a strong adaptability in the complex work environment.