An optical touch system includes two optical sensing modules and a processing module. The optical sensing module includes a mounting base and a light sensing device. The mounting base has an alignment structure and is disposed at a corner of a touch area by aligning the alignment structure with the profile of the corner. The light sensing device is disposed on the mounting base toward the touch area. The processing module receives signals from the two optical sensing modules to generate coordinates relative to a touch performed on the touch area. A touch display system using the optical touch system further includes a displaying device. The processing module of the touch display system also controls the displaying device to display an image on a screen according to the coordinates; therein, the screen thereon defines the touch area.