A supporting plate for an automatic document feeder, adapted to configure a plurality of paper feeding rollers installed correspondingly to a plurality of transmission wheels, and used for transmitting a document to a paper output tray, the supporting plate including a plate body, including a plurality of accepting holes and adapted to install the plurality of paper feeding rollers, the plurality of paper feeding rollers and the plurality of transmission wheels drive the document move along a paper output direction toward the paper output tray; and a plurality of supporting portions, disposed at sides of the plurality of accepting holes along the paper output direction, and used for supporting the document to cause the document to form a curved body while being moved. A paper output mechanism using the aforementioned supporting plate is also disclosed here.