An optical touch system includes a screen, an image-erasing device, an image-sensing module, a processing unit, and an image processor. The image-erasing device includes a body and reflection portions on the body. The image-erasing device is detachably contacted with the screen. The image-sensing module includes a light source emitting a light beam to the reflection portions to respectively generate a reflected light beam, and an image sensor detecting the reflected light beams. The processing unit obtains positions on the screen respectively corresponding to the reflection portions according to the reflected light beams, and defines position information of an erasing region according to the positions on the screen. The image processor outputs a display image to be displayed on the screen, and erases data of the display image corresponding to the erasing region according to the position information.