In order to improve the welding quality and efficiency of the rectangular fillet weld in the shipyard and the steel structure workshop, reduce the labor cost, and improve the welding automation, it is necessary to study a welding robot that can track rectangular fillet weld. The working principle of the rotating arc sensor has been studied, and the mathematical model of the space posture of the arc welding gun has been established. The equivalent link coordinate systems of the wheeled mobile robot have been built, and the jacobian matrix of the robot and its inverse matrix have been calculated. The transformation from the operational space speed to the joint space speed has been realized by using the inverse matrix of the jacobian matrix, and the trajectory planning of the welding robot has been finished. The tracking algorithms of the linear fillet weld and the rectangular fillet weld have been studied, and the rectangular fillet weld tracking experiment has been done in the laboratory and the factory. Experimental results showed that the welding robot can track the rectangular fillet weld with high accuracy and good reliability.