A scanning module capable of finely tuning the optical length comprises a lamp tube, a base, a photosensitive substrate, and an adjustment unit. The lamp tube provides a required light source for a document to be scanned. The base has a reflective mirror set and a camera lens. The reflective mirror set is used to receive light from the document, and transmits the light to the camera lens for focusing. The photosensitive substrate is used to convert light outputted by the camera lens into electronic signals. The adjustment unit is used to adjust the optical length between the base and the photosensitive substrate. The total optical length can thus be adjusted to correct the inaccuracy of magnification. Or the distance between the camera lens and the photosensitive substrate can be adjusted to align the focal length within the allowable inaccuracy of magnification to achieve an output image of better quality.