The invention discloses a narrow-gap semi-automatic gas protection welding gun which comprises a gun barrel, a connecting rod, a tip, a nozzle adapter and a conical nozzle. The rear end of the connecting rod is connected with the front end of the gun barrel. The front end of the connecting rod is connected with the tip. Channels communicating with the gun barrel are respectively arranged in the connecting rod and the tip. The nozzle adapter is arranged on the outer surface of the rear end of the connecting rod. The conical nozzle comprises a connecting end and an operating end. The conical nozzle is connected with the nozzle adapter by means of the connecting end, and sleeves the outer parts of the connecting rod and the tip. The operating end adopts a flat tubular structure. Compared with the prior art, the narrow-gap semi-automatic gas protection welding gun has the advantages that the structure is simple, use is convenient, operation is simple and convenient, the cost is low, the generality is high, the gas protection welding gun has no special requirements on welding machines, is matched with a common gas protection welding device, and can be applied to welding of a narrow-gap welding seam of which the depth is smaller than 120 mm and the width is larger than 10 mm.