A pneumatic nail gun includes a gun body, a muzzle connected to the gun body for receiving a nail, a cylinder unit disposed in the gun body, an air valve unit having a cover portion, and a striking unit having a piston. The cylinder unit includes a gas storage chamber, a striking cylinder having a cylinder chamber 510 in fluid communication with the gas storage chamber, and at least one gas storage cylinder in fluid communication with the gas storage chamber for receiving a gas having a pre-determined gas pressure. The cover portion is operable to interrupt fluid communication between the cylinder chamber and the gas storage chamber, and to allow the fluid communication such that the piston is pushed by the gas to strike the nail.