1. A compensation apparatus for image scan, applied to an optical scanner that comprises a platform to locate an object to be scanned thereon, a photosensitive apparatus with a set of scan photosensitive devices, and a storage apparatus, wherein a scanned image is obtained and temporarily stored in the storage apparatus after the object is scanned, the compensation apparatus comprising: a set of calibration boards, having two calibration boards located at two sides of the platform; a set of calibration photosensitive devices, located at two sides of the set of scan photosensitive devices to obtain a set of calibrated images by detecting the calibration boards; and an image processor, to extract and compare the calibrated images for adjusting the scanned image.