A sheet-fed scanner capable of scanning multiple scan lines for image signal calibration includes a housing, a sheet-feeding mechanism, a first scanning module, a first actuator and a first calibration sheet. The sheet-feeding mechanism disposed in the housing feeds a document across a scan region. The first scanning module rotatably mounted in the housing scans a front side of the document fed across the scan region. The first calibration sheet is fixed in the housing and disposed in the scan region. The first actuator drives the first scanning module to rotate such that the first scanning module senses a plurality of scan lines on the first calibration sheet to obtain a first calibration standard for calibrating a first image signal of the front side of the document. Thus, it is possible to avert any flaws in image quality caused by the contaminated calibration sheet.