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 light and an image of the object reflecting light, the object detection method comprising the steps of: acquiring a first image and a second image, wherein the first image has a brightness information and is formed by the object blocking the light and the second image has an image feature and is formed by the object reflecting the light; calculating a representative brightness of every column of pixels in the first image according to the brightness information; determining an object range in the first image according to the representative brightness; calculating the image feature associated with the object range in the second image; and identifying a block information of the object within the object range according to the image feature associated with the object range.