A pneumatic nail gun includes a gun body with a trigger, collecting a compressed high pressure air with a constant pressure, a movable cylinder installed in the gun body, a hitting piston disposed in the cylinder, a valve ring formed outside the cylinder, a head valve disposed on the top of the cylinder, at least one return passage formed outside the cylinder, and a valve bolt disposed in the return passage. Before the trigger is pressed, the high pressure air drives the valve ring to keep the cylinder at the upper position to close the head valve. When the trigger is pressed, the high pressure air drives the cylinder move downwardly to open the head valve for guiding the high pressure air to drive the piston downwardly move to hit nails. When the trigger is released, the high pressure air drives the valve bolt open for guiding the high pressure air via the return passage to drive the piston rapidly move upwardly to reposit.