The invention discloses a welding gun cylinder. The welding gun cylinder comprises a first cylinder body and a second cylinder body. The first cylinder body comprises a first cylinder barrel, a first piston and a first piston rod. The second cylinder body comprises a second cylinder barrel, a second piston and a second piston rod. A first middle end cover is arranged between the first cylinder barrel and the second cylinder barrel. The second piston rod penetrates the first middle end cover and is connected with the first piston rod. A built-in cylinder is slidably arranged in the second cylinder barrel. The second piston is slidably arranged in the built-in cylinder. A first ventilation pipeline is arranged in the second piston rod. A rear end opening of the first ventilation pipeline is located between the second piston and a built-in cylinder rear end cover of the built-in cylinder. A front end opening of the first ventilation pipeline is located between the first piston and the first middle end cover. According to the welding gun cylinder, it is not necessary to supply air to the built-in cylinder separately, and the structure is simplified; when air enters a second air exchange opening, the double-acting force is provided, so that the length of the cylinder is reduced, and the size of the cylinder is reduced. The invention discloses a welding gun cylinder system and an operation method of the welding gun cylinder system.