A calibration mechanism for an optical module of a sheet-fed scanner is disclosed. The calibration mechanism includes a calibration strip and a driving unit. The calibration strip is moved by the driving unit, and the calibration strip may be moved relative to the optical module for calibrating color depth. The driving unit includes a roller and a flexible strip. One end of the flexible strip is mounted to the roller, and the calibration strip is mounted or formed on the flexible strip. The flexible strip is moved by the roller action so that the calibration strip is moved relative to the optical module, and the optical module may accordingly calibrate the color depth.