A paintball gun triggering mechanism includes a front barrel, a rear barrel, a triggering barrel and a rear guide rod. A front end of the front barrel comprises a front ring formed inwardly and a retreating air inlet is provided behind the front ring. A front end of the rear barrel is fixedly connected to a rear end of the front barrel, and a retreating air outlet, a rear ring and a launching air inlet are provided in sequence behind the connecting portion of the front barrel and the rear barrel. The triggering barrel comprises a center ring formed at a center outer portion thereof and is placed in the front barrel and the rear barrel. The rear guide rod has a piston at a front end thereof to be inserted into the triggering barrel, and has its rear end secured to the rear end of the rear barrel.