A beam combiner and a projection system are disclosed. The projection system includes at least two light sources, a beam combiner, a light tunnel, a light modulator, and a screen. The beam combiner includes at least two prisms. Each prism includes a reflective surface and at least a combining surface. The reflective surface is coated with a high reflective coating and each prism is combined with another prism at the combining surface. The light modulator produces image signal. Light produced from the at least two light sources is reflected by the beam combiner, and enters the light tunnel and the light modulator sequentially, and the screen receives the image signal from the light modulator.