An optical touch system and a touch point calculation method thereof are provided. The optical touch system includes a first lens, a second lens and a screen. The first and second lenses are disposed on the same side of the screen and face to the other side of the screen. In the method, images of a touch object above the screen are captured and used to calculate a first and a second touch signal of the touch object touching the screen before and after a preset time. Then, it is determined whether a width difference of the first and second touch signals exceeds a predetermined threshold. If yes, a third touch signal corresponding to the first touch signal relative to the second touch signal is calculated. Finally, positions of the first touch signal and the third touch signal are taken as locations of two touch points on the screen.