A paper-feeding mechanism of an automatic document feeder includes a pick-up module, a supporting module and a paper stopping module. The pick-up module is used for picking an uppermost paper sheet from the stack of paper sheets and transporting the uppermost paper sheet into a passageway inside the automatic document feeder. The supporting module supports the pick-up module and is rotated to either ascend or descend the pick-up module, wherein the supporting module has a sustaining part on a sidewall thereof. When the supporting module and the pick-up module are downwardly rotated, the stopping plate is moved vertically to a paper-feeding position for allowing the uppermost paper sheet to be transported into the passageway. When the supporting module and the pick-up module are upwardly rotated, the stopping plate is moved vertically to a paper-stopping position for hindering the uppermost paper sheet from being transported into the passageway.