A scanning device includes a first chassis, a second chassis and a calibration mechanism. The first chassis is for scanning one side of a document. The second chassis is for scanning the other side of the document. The second chassis is movably disposed opposite the first chassis. The calibration mechanism is for calibrating the color depth of the scan image. The calibration mechanism includes a calibration sheet and an elastic member. One end of the elastic member is fixed in the scanning device. The second chassis exerts a force on the calibration sheet to generate a relative movement between the calibration sheet and the first chassis. When the second chassis ceases exerting the force, the elastic member releases a resilient force for moving the calibration sheet to a starting position. The first chassis performs dynamic calibration by the relative movement between the first chassis and the calibration sheet.