A compensation apparatus for image scan, applied to an optical scanner with a platform, on which an object to be scanned is disposed. The optical scanner has a photosensitive apparatus with a set of scan photosensitive devices and a storage apparatus. When the object is scanned by the set of scan photosensitive devices, a scanned image is obtained and saved in the storage apparatus temporarily. The compensation apparatus has a set of calibration boards, a set of calibration photosensitive devices and an image processor. The set of calibration boards has two calibration boards located at two sides of the platform. The set of calibration photosensitive devices is located at two sides of the set of scan photosensitive device. The image processor is used to extract and compare the calibrated image, so as to adjust the scanned image.