A firing mechanism for a paintball gun is received in the path of the barrel and a trigger is used to activate the firing mechanism. The firing mechanism includes a driving member, a switch member, a control member and an inner part, all of which are received in the path of the barrel. The driving member drives the switch member to change the air flow in the marker so as to move the control member axially to control the high pressurized air to move the inner part such that the high pressurized air is released to eject the paintballs out from the barrel.