Based on the analysis of the welding of flat steel,an automatic welding system is designed by using the XDM PLC as the main controller,and the three-axis motion mechanism and the R-axis rotation mechanism as actuators. According to working requirements,such as four-dimensional space degree of freedom welding requirements,system hardware configurations,the actuator design,system hardware selection,and the hardware connection form of the system are determined at first. In this paper, the composition and working principle of the automatic welding system are introduced, focusing on the coordinate transformation relationship in the visual measurement,and the calibration process is given. The compensation methods after the rotation of the welding gun at each inflection point are discussed,and the different welding parameters are used according to different seam types during the welding process. The flow chart of control program is given out for the welding gun attitudes adjustment. The welding test result shows that this designed system can track the welding seam quite well and welding quality is satisfying,the more,this system can improve the welding efficiency greatly.