A pneumatic nail gun includes a gun body having a movable cylinder therein; a hitting piston disposed in the cylinder; a main air housing collecting a compressed high pressure air with a constant pressure; and at least one main passage disposed at a peripheral portion of the cylinder or a side of the main air housing, the at least one main passage having a valve bolt therein to divide the main passage into a top passage, a middle valve chamber, and a bottom passage, the top passage and the middle valve chamber being connecting with the main air housing for introducing and gathering high pressure air therein, the middle valve chamber being connecting with the bottom passage via the main valve opening, the valve bolt being capable of controlling the opening and closing of the main valve opening for rapidly repositing the piston.