This paper proposes a path-planning algorithm using image processing for three-dimensional (3-D) arc welding of the automatic welding robot system. First, the forward kinematics of the automatic welding robot system is analyzed. Then, the path-planning algorithm based on image processing is developed, and its script is described. Furthermore, the practical automatic welding robot system is presented, and the 3-D arc welding is carried out experimentally. The interaction interface of the image processing subsystem to display actually welding point and the desired welding point is used. Experimental results demonstrate that the tracking of 3-D welding seam can be successfully achieved, and the image of actually welding point tracks the desired welding point on the interaction interface. The welding trajectory can be automatic obtained based on the script algorithm using data generated by the image process subsystem. The proposed path-planning algorithm is verified.