1. A method of enhancing scan resolution, suitable for use in a scanner with an optical sensor, wherein a range that a detecting cell of the optical sensor can detect includes a plurality of original pixels with a predetermined number, and a result of one detection by the detecting cell is called a scanned pixel, the method comprising: scanning a smooth image region, wherein the smooth image region includes at least the original pixels with the predetermined number and has a uniform brightness, to obtain a smooth image data; and processing scanned images obtained by scanning a document according to the smooth image data.