There is provided an optical distance detection system which includes a light source and a detection device. The light source is configured to illuminate a surface of an object. The detection device is configured to receive a reflected light from the surface of the object and to output a distance of the object according to the reflected light. The detection device includes a sensing module and a calculation module. The sensing module is configured to receive the reflected light to accordingly generate an image. The calculation module is for outputting the distance according to a light spot position and a light spot size of the reflected light forming on the image.