This paper presents an automatic welding machine used for the carbon dioxide gas welding, with the mechanical structure designed, and the working process of the automatic welding machine analyzed. With PLC control system design method, we gave the system's control circuit principle diagram and I/O address assignment table, and carried out the design of hardware and software, drew the system's flow chart and elaborate on it. The Human Machine Interface (HMI) of the system is also designed. Experimental operation results show that PLC-controlled automatic welding machine can improve welding quality and efficiency, reduce labor intensity and bring huge economic benefits.