1. A sheet-fed scanner, comprising: a scan channel; a feeding unit disposed at an inlet of the scan channel for feeding a document; a transmitting unit disposed in the scan channel for transmitting the document to pass through the scan channel; a background sheet disposed on a side of the scan channel and having a first surface facing towards the scan channel and a second surface; a background light source facing towards the second surface to provide an intermittent light signal to the background sheet, which correspondingly displays an intermittent variation background image on the first surface; and a scan module disposed on another side of the scan channel, wherein when the document passes through the scan channel between the scan module and the background sheet, the scan module captures a document image of the document and the intermittent variation background image to generate scan data.