An optical touch screen system includes a touch screen, an image-sensing device, and a processing device. The image-sensing device generates a plurality of light-shielding information of an object at a plurality of time points. The processing device coupled to the image-sensing device includes a computing unit calculating a projected size difference value according to the plurality of light-shielding information, wherein the projected size difference value is a difference value between the projected sizes of the object on the touch screen at two different time points.