An optical system for a projector is provided, which includes a solid-state light source module, an optical module, a phosphor element and a light relay module which are disposed sequentially. The light source module can generate a first beam, while the optical module can relay and uniformize the first beam so that the first beam is uniformly incident on the phosphor element. The optical module also makes the light spot of the first beam incident on the phosphor element into a predetermined shape and area, so that the phosphor element will emit a second beam by an emitting area. The light relay module can receive a part of the second beam, and the part of the second beam has a solid angle. An Etendue defined by the solid angle and the emitting area may be identical to the Etendue of a light modulator of the projector.