A sheet-fed scanner comprising a scan channel, a feeding unit, a transmitting unit, a background sheet, a background light source and a scan module is provided. The feeding unit is disposed at an inlet of the scan channel for feeding a document. The transmitting unit transmits the document to pass through the scan channel. The background sheet is disposed on a side of the scan channel, and has a first surface and a second surface. The first surface faces towards the scan channel. The background light source faces towards the second surface to provide an intermittent light signal to the background sheet, so that the background sheet correspondingly displays an intermittent variation background image on the first surface. The scan module is disposed on another side of the scan channel, and captures a document image of a document and the intermittent variation background image to generate scan data accordingly.