A nail gun includes a cylinder, a main chamber, a piston slidably disposed in the cylinder, and a trigger valve disposed in the nail gun. The piston divides the cylinder into a top air chamber and a bottom air chamber, and the trigger valve can actuate the high pressure air in the main chamber to drive the main air valve to open or close the fluid communication between the main chamber and the top air chamber. A main air valve is disposed at a top portion of the cylinder, which includes a valve port, a top and bottom air tight rings disposed on the piston. The valve port is disposed between the top and bottom air tight rings when it is closed, and moves to a place away from the top air tight ring when it is opened.