A scanning method in which a scanning head moves back and forth to scan a paper sheet located in a scanning window is disclosed. The method of the present invention includes the steps of a) making the paper sheet go forward a first distance into the scanning window; b) the scanning head scanning a portion of the paper sheet; c) the transmission mechanism driving the scanning head to move a second distance in a first direction, wherein the second distance is smaller than the first distance; d) repeating the steps b) and c) until the scanning head completely scans a plurality of portions of the paper sheet in the scanning region; e) repeating the steps a), b), c) and d), except that the scanning head moves in a second direction opposite to the first; f) repeating the steps a), b), c), d), and e), the movement of the scanning head alternating between a first and second direction, until the scanning head completely scans the paper sheet to be scanned.