An improved pneumatic device includes a body having a hollow cavity, defining an X axis and having a first end and a second end. The body includes a movable cylinder, a nail-firing piston rod and at least one exhaust hole. The movable cylinder and the nail-firing piston rod reciprocate along the X axis in the body by performing a forward stroke to the second end, and a return stroke back to the first end. The movable cylinder presents an open status when completing the forward stroke and presents a closed status when completing the return stroke. The movable cylinder has an exhaust hole and an exhaust valve disposed at an outer periphery thereof. When the nail-firing piston rod has not completed the return stroke yet and the cylinder is in the closed status, an exhaust channel is thereby formed to facilitate exhausting of air.