A switch mechanism for a stapler includes a trigger, a pivotable member, and a swing member, which is pivotably connected to the trigger by two pins. The pivotable member has a first protrusion extending from a top of the pivotable member. The pivotable member is received in a recess of the slide member and a second protrusion extends from an inner top end thereof A spring is connected between the first and second protrusions so that the slide member is movable relative to the pivotable member. The swing member includes a first curved part and a second curved part which is located at a side of the pivotable member. The curved part of the swing member is inserted into a hole of the slide member when pulling the trigger, and the slide member is moved downward to change the relative position between the slide member and the valve of the stapler.