Abstract: The automatic control method of open arc surfacing welding system was studied for the milling roller.The moving system formula was built according to the mechanical structure of the open arc surfacing welding system.The surfacing welding process could be divided into same-arc section and transition-arc section according to its characteristics.Fitting the sunk milling roller as a spherical surface, a moving-track control method was raised and the moving control formulas for the same-arc section and the transition-arc section were derived.The error formula was also built.The error analysis gained with the aid of MATLAB proves the formulas and it meets the technical requirements.