A pneumatic firing device includes a rear section engaging a cylinder, a solenoid valve engaging the rear section from under, an adjusting element disposed inside the rear section, a piston disposed inside the cylinder to form a first chamber and a second chamber, an airflow guiding element engaging the cylinder at a front thereof, a moving rod having a tail end engaging the piston and a front end stretching through the cylinder into the airflow guiding element for operation control of the airflow guiding element, and a front section engaging a front of the airflow guiding element and having a plurality of passage to displace the delivery tube through a hole thereof. The solenoid valve holds control of the operation of a pressure release channel to change the pressure difference between the first and second chamber, making the pressured air entering a first passage from a first guiding hole of the airflow guiding element, pushing the delivery tube to a firing position, flowing into a second passage from a second guiding hole, and then firing a pellet with strong airflow.