1. An object detection method for an optical touch system, the optical touch system identifying at least one object according to an image of the object blocking invisible light and an image of the object reflecting visible light, the object detection method comprising: acquiring a first image and a second image, wherein the first image has a brightness information and is formed by the object blocking the invisible light, and the second image has an image feature and is formed by the object reflecting the visible light, wherein the first image and the second image are respectively acquired by different partial areas of a same sensing array; calculating a representative brightness of every column of pixels in the first image according to the brightness information; determining an object range of the object in the first image according to the representative brightness; calculating the image feature within a pixel range in the second image after the object range in the first image is determined, wherein a width of the pixel range in the second image is determined corresponding to a width of the object range in the first image; and identifying a block information of the object within the object range according to the calculated image feature within the pixel range in the second image.