The present invention relates to an automatic sheet-feeding scanning apparatus having a movable guide rib device. The automatic sheet-feeding scanning apparatus includes a flatbed scanner and an automatic document feeder. The movable guide rib device is mounted in a housing of the flatbed scanner and includes a guide rib structure, at least one hollow portion, a plurality of elastic members and a fixing plate. The guide rib structure is movable upwardly or downwardly so as to avoid document damage even if the document is relatively large-sized.