The invention discloses a plate flattening device. The plate flattening device comprises an upper pressing head, a lower pressing head and a plate fixing device, wherein the upper pressing head and the lower pressing head are fixed on a frame; the upper pressing head is arranged right above the lower pressing head; the plate fixing device is arranged between the upper pressing head and the lower pressing head; the plate fixing device comprises a first guide rail, a second guide rail and a fixing platform; the first guide rail and the second guide rail perpendicular to each other are arranged from bottom to top; the fixing platform is capable of moving along with the second guide rail; a locking device is arranged on the top surface of the fixing platform; the positions required to be flattened are flattened by the plate flattening device or the upper pressing head and the lower pressing head completely flatten the plate. The plate flattening device is capable of carrying out fixed-point flattening on the plates, so that the target requirements are met; meanwhile, the flatness of the plates can be automatically detected; parts required to be flattened are moved between the upper pressing head and the lower pressing head to be flattened; the flattening effect can be automatically detected until the flattening effect is qualified; the full-automatic flattening process is achieved; the flattening efficiency is improved; the labor intensity is reduced.