A pneumatic transmission device includes a transmitting channel, an air inlet and an air supply device. The transmitting channel includes a first feeding path, a second feeding path, and a compressing path communicated between the first feeding path and the second feeding path. The compressing path is gradually inclined downward and rearward from a front end of the compressing path to a rear end of the compressing path. The second feeding path is equipped with a scanning unit. The air inlet is slantwise extended upward and frontward from a top wall of the transmitting channel. An inside of the air inlet defines a passageway. The passageway is connected with the second feeding path. The air supply device is connected to the passageway. The air supply device injects high pressure air into the passageway.