A scanner includes a housing, a transparent platen disposed on the housing for positioning a document, and a calibration pattern formed on a side of the transparent platen. The calibration pattern includes an arc having a first point and a second point. The scanner further includes an optical engine installed inside the housing for scanning the document and the calibration pattern with movement in a first direction, and a control module installed inside the housing for controlling the optical engine to move to a scan start position according to the first point and the second point of the calibration pattern scanned by the optical engine and a radius of curvature of the arc.