The invention relates to a plasma and gas metal arc welding composite welding device with a simple structure. The device is formed by cutting off 25mm to 35mm of the lower part of a welding gun nozzle of a general gas metal arc welding machine and is assembled on a welding gun by replacing the nozzle of a welding gun of the general gas metal arc welding machine. The nozzle on the welding gun is connected with a bakelite insulating layer by a thread pair; the inner wall of the bakelite insulating layer is connected with a copper electrode water cooling tank; the lower end of the copper electrode water cooling tank is connected with a copper electrode; the outer wall of the lower end of the bakelite insulating layer is connected with a restraining nozzle; the outer wall of the lower end of the restraining nozzle is connected with a protective nozzle; a conductive nozzle clamp is coaxially arranged inside the nozzle on the welding gun; the lower end of the conductive nozzle clamp is connected with a conductive nozzle; a welding wire sequentially passes through the conductive nozzle clamp, the conductive nozzle, the copper electrode and the restraining nozzle from the axis of the welding gun. The device disclosed by the invention has the advantages of being capable of increasing the cladding rate of welding of the gas metal arc welding machine, realizing a more stable welding process and being simpler in structure and lower in cost.