A pneumatic nail gun includes a gun body, a movable cylinder, a piston and a slidable sleeve valve. The gun body has a main air housing collecting a compressed high pressure air with a constant pressure, and a trigger at one end of the main air housing driving the high pressure air to shoot a nail. The movable cylinder is disposed in the main housing, which moves upward when the trigger is pressed, and moves downward for reposition when the trigger is released. The piston is slidably movably disposed in the cylinder, which has a driver blade extending from the piston. The driver blade protrudes out of the gun body for punching against the nail in accordance with a downward movement of the piston. The slidable sleeve valve is assembled at an outer peripheral surface of the cylinder, which introduces high pressure air to drive the piston move upward when the cylinder moves downward to its lower dead center, and is driven downward after the piston moves to its upper dead center.