A reflective light guide includes a cylinder enclosed by more than three reflecting plates which connected with each other in turn. The reflecting plate includes a plate body and a reflecting layer arranged on the plate body, along the exit direction of light. The cylinder includes a light inlet part, a light reflecting part and a light outlet part in turn, wherein the light outlet part larger than the light inlet part such that the cylinder being cone shaped. The LED light source with a reflective light guide tube includes a LED light source and a reflective light guide tube. The optical system includes a reflective light guide tube, a LED light source and an optical assembly.