A flat spring plate includes an outer closed-loop frame having a plurality of evenly spaced outer connection parts and a plurality of elongated side frame members each interconnected between the two corresponding adjacent outer connection parts, an inner closed-loop frame concentric with and surrounded by the outer frame, and a plurality of spring parts interconnected between the respective outer connection parts of the outer frame and the respective inner connection parts of the inner frame. The inner frame includes a plurality of evenly spaced inner connection parts. Each of the spring parts includes a U-shaped part adjacent to the corresponding outer connection part, a curved part adjacent to the corresponding inner connection part, and an elongated intermediate part interconnected between the U-shaped part and the curved part. Each elongated intermediate part is substantially parallel to the corresponding side frame member of the outer frame.