A driving device for resetting a hitting nail bar of the pneumatic nail gun, which is mounted in a gun body including a main air chamber, a trigger valve, a hitting nail bar and a bottom cylinder chamber disposed therein is provided. The driving device includes a main gas channel and a sensing valve component slidably disposed in the main gas channel. The main gas channel communicates with the main air chamber and the bottom cylinder chamber via the trigger valve. The sensing valve component is controlled by the position of the hitting nail bar to be either opened or closed. The sensing valve component is opened on the condition that a reset state of the hitting nail bar is excluded, and the high-pressure air is introduced from the main air chamber into the bottom cylinder chamber to drive the hitting nail bar to move upwardly to reset rapidly.