A calibration mechanism is positioned in a scanning device having a scanning module for scanning a document. The scanning module defines a scanning line perpendicular to a scanned area of the document. The calibration mechanism includes a calibration unit, a supporting unit supporting the calibration unit, and a driving unit. The calibration unit is positioned to face the scanning module and aligned with the scanning line. The driving unit drives the calibration unit to move close to the scanning module and away from the scanning module along the scanning line. The scanning module obtains great brightness value by scanning the calibration unit, if the calibration unit is close to the scanning module. The scanning module obtains small brightness value by scanning the calibration unit, if the calibration unit is far from the scanning module.