Abstract: A color light mixing device, a color light mixing method and a small-sized projecting system using the color light mixing device are provided. The color light mixing device includes a tube structure, a plurality of light-emitting units, a reflective polarizer and a microlens assembly. The light-emitting units are located at an inlet end of the tube structure. The reflective polarizer is located at the other end of the tube structure. The microlens assembly is arranged between the light-emitting units and the reflective polarizer. By the color light mixing device, the light beams with a single polarization state are homogenized and outputted to a microdisplay element of the small-sized projecting system. By an optical projection lens assembly of the small-sized projecting system, an image shown on the microdisplay element is projected onto a screen.