The invention discloses an automatic thin knife longitudinal cutting device for paperboard. The device comprises a rack, wherein conveying rollers are rotatably arranged at two ends of the rack, a conveying belt is rotatably mounted on the surfaces of the conveying rollers, a mounting plate is fixedly mounted at one end of the rack, fixing plates are fixedly connected to the surface of the other side of the mounting plate, an inclined connecting plate is hinged to the surface of one side of each fixing plate through a hinge seat, the other end of each inclined connecting plate is fixedly connected with a mounting frame, the side surface of each mounting frame is rotatably provided with a longitudinal cutter, the shaft end of each longitudinal cutter is fixedly provided with a driven belt wheel, the surface of the other side of each mounting frame is fixedly connected with connecting springs, the other ends of the connecting springs are fixedly provided with auxiliary pressing plates, and the side surfaces of the auxiliary pressing plates are provided with a strip cutting groove. In this way, the convenience and stability of use can be greatly improved, and the safety and high efficiency can be ensured.