Automatic paper shredder having a paper pressing mechanism, which includes upper shredder cover, lower shredder cover, paper support box, paper pressing plate, cover plate, spindle and two spindle holders. Upper cover is connected to lower cover. Paper support box is above the lower cover forming a cavity for to-be-shredded paper. Fixed ends of paper pressing plate and cover plate are around the spindle, so free ends of paper pressing plate and cover plate can rotate coaxially. Two ends of spindle are respectively disposed in two spindle holders, so spindle can rotate. Two spindle holders are connected to upper cover to connect cover plate and paper pressing plate to upper cover. Paper pressing plate is disposed below cover plate and for pressing to-be-shredded paper placed on paper support box when a user closes cover plate.