A projection device includes a prism set, a digital micro-mirror device, a reflective component and a heat dissipation module. The prism set includes a light-incident side and a mirror-corresponding side opposite to each other, and includes a first light-emitting side and a second light-emitting side adjacent to each other. The digital micro-mirror device spatially corresponds to the mirror-corresponding side. An incident light is transmitted through the light-incident side to irradiate the digital micro-mirror device. When the digital micro-mirror device generates an on-state light, the on-state light is outputted through the first light-emitting side. When the digital micro-mirror device generates an off-state light, the off-state light is transmitted toward the reflective component and outputted through the second light-emitting side. The heat dissipation module spatially corresponding to the second light-emitting side is configured to absorb and convert the off-state light for heat dissipation.