A paintball gun percussion mechanism includes a barrel, a propeller, and a bolt. The barrel contains a first air chamber and a second air chamber connecting through a first airway and a second airway. The propeller is inserted into the barrel. The bolt is provided with a plate insert to relatively seal the propeller. A retaining wall provided on the propeller reciprocally moves in the first chamber to launch a projectile by taking in or out air in the first and the second airways.