The invention discloses a gun body of a welding gun of a welding head. The gun body of the welding gun comprises a gun body casing, and the gun body casing is a hollow casing and is flat. A cooling system and a primary gas protection system are arranged in the gun body casing, the gun body casing is further provided with a tungsten electrode system, and the tungsten electrode system penetrates the gun body casing from the middle. A tungsten electrode in the tungsten electrode system is exposed out of the front end of the gun body casing, and the tungsten electrode on the tungsten electrode system is rotatable. The welding head is infinitely rotary, the gun body of the welding gun on the welding head can implement welding on a circumferential weld joint around the welding head in the rotating process of the welding head, and the welding quality can meet the quality requirements of nuclear power products.