An optical sensing system includes a sensing area, a reflective mirror, a first image-sensing device, a second image-sensing device, and a processing circuit. The sensing area is an area in which a plurality of pointing objects may execute touch operation. The reflective mirror generates a mirror image of the sensing area. The first image-sensing device and the second image-sensing device respectively capture images including all or part of the pointing objects in the sensing area, and all or part of the pointing objects in the reflective mirror. The processing circuit generates candidate coordinates according to the images captured by the first and the second image-sensing devices, and obtains the locations of the pointing objects from the candidate coordinates by means of the symmetric relationship between the point objects and the corresponding mirror images with respect to the reflective mirror. In this way, the optical sensing system can perform multi-touch operation.