Abstract: As the requirement of high accuracy in seam tracking,the systems dynamics become more prominent in seam tracking process. The non-complete dynamic model for the four wheeled mobile welding robot was established. Variable structure controller with sliding mode has been designed to complete coordinately controlling of cross-slider and mobile welding robot. Several factors have been considered including the influence of the inertia,electrical machinery dynamics,surface irregularity and other factors in seam tracking process on the tracking accuracy. Through the theoretical analysis and simulation,it has been proved that the dynamic model and the variable structure controller with sliding mode are correct and valid in seam tracking. It has set up important foundation for off-line programming,simulation and dynamic control of mobile welding robot.