A method for bonding a backsheet is disclosed. The bonding is performed in two steps to ensure the bonded flexible master plate has a uniform size. The first pressure in the first bonding is smaller than the second pressure in the second bonding. A relatively small pressure is used in the first bonding to attach the backsheet to the back surface of the flexible master plate, so that the backsheet is subject to small deformation and the uniformity in size is ensured. The pressure is increased in the second bonding to exert sufficient force on the backsheet, increase firmness of attachment, and ensure that the bonded flexible master plate has a uniform size.