A calibration mechanism of a scanning device including an assembly of a flexible member and a calibration strip and a limiting member is disclosed. The assembly of the flexible member and the calibration strip can travel in reciprocating motion, and the limiting member limits the distance of the vertical drift of the calibration strip. Thus, the calibration strip is confined within an anticipated position, and the accuracy and the consistency of the calibration results for an optical module of the scanner, which acquires image data of a plurality of the-scanning lines of the calibration strip, are greatly improved.