A projection system and a light tunnel applied in the projection system. The light tunnel comprises a first reflector, a second reflector, a third reflector, a fourth reflector, a light entrance, and a light exit. A distance between the third and fourth reflectors gradually changes from the light entrance to the light exit. The distance between the third and fourth reflectors at the light entrance and the light exit is less than length of each of the first and second reflectors. Thus, the light tunnel can be easily held and applied to various projection systems using various light sources and image generation devices.