A projection device is disclosed. The projection device includes an air inlet, an air outlet, an optical engine, a heat sink, a fan and a combiner. A first passage is arranged between the air inlet and the air outlet for passing an air flow. The optical engine includes a light source and an imaging lens set outputting a projection light beam. The heat sink is connected to the optical engine, and a part of the heat sink is arranged in the first passage. The fan is arranged in the first passage. The combiner receives the projection light beam. The projection light beam moves in a propagating direction opposite to the entry direction.