The present application provides a back plate structure and a flexible display device. The back plate structure includes: a first back plate, a second back plate and a flexible shaft. The first back plate and the second back plate are connected by the flexible shaft. The flexible shaft includes an arch component and at least one end component. The arch component includes a concave surface and a convex surface corresponding to the concave surface, and the concave surface faces a bending direction of the flexible display device. Each end component includes at least one foldable structure, and each foldable structure includes two planes parallel to a bending axis of the flexible display device. Each end component is located at an end of the arch component and obscures a space formed by the arch component.