An optical system for a projection device is disclosed. The projection device includes three different light sources, three condensers corresponding to the light sources, an integrator, an optical lens array, a total internal reflection prism, a digital micro-mirror device and a projection lens. The light sources generate light transmitting through the condensers, the integrator, the optical lens array, the total internal reflection prism, and the digital micro-mirror device into the projection lens. The optical system includes an emitting and reflecting module emitting and reflecting light from the light sources to the integrator and comprises a first, second emitting and reflecting mirrors intersecting thereto, the intersections of the first, second emitting and reflecting mirrors facing the corresponding light sources and condensers.