An operation method of an optical touch device includes: emitting, by a light emitting unit, a light beam to illuminate an object; capturing, by an image sensing device, an image of the object reflecting the light beam; selecting all pixels in the image having a brightness greater than or equal to a brightness threshold; sorting the selected pixels along a first coordinate axis of the image, a second coordinate axis of the image or based a pixel brightness; selecting the top first predetermined ratio of pixels from the sorted pixels as an object image of the object; and calculating a gravity center of the object image according to positions of the top first predetermined ratio of pixels or according to the positions of the top first predetermined ratio of pixels with a weight of pixel brightness. An optical touch device is also provided.