Sheet folding apparatus, image forming apparatus using the same, and sheet folding method

The present invention provides a paper folding device, an image forming device with the paper folding device and a paper folding method, wherein, the paper folding device comprises the following components: an intermediate bookbinding unit which is used for binding the central parts of paper piles; an intermediate folding unit which is used for folding the central parts and forming a folding mark; a bearing platform which is used for bearing the paper piles conveyed from the intermediate folding unit; a clamping plate which is configured to parallely press the bearing platform and separate with the bearing platform for clamping the paper piles on the bearing platform; and a first roller and a second roller which are used for clamping and pressing the folding mark of paper piles clamped by the clamping plate and simultaneously moving along the direction of folding mark and strengthening the folding mark, wherein, the surface of clamping plate facing to the bearing platform is installed with an elastic component.