The invention relates to a photothermal power generation composite material backplane and a preparation method thereof. The composite material backplane includes a backplane and a reflective mirror surface laid on the backplane, and the backplane is provided with a reinforcement integrally formed with it. Ribs and a plurality of connecting discs, the connecting discs are tightly connected with the reinforcing ribs, and the back plate and the connecting discs are both made of epoxy resin. Firstly, the connecting discs are prepared in the mold, and then on the back In the plate mold, the reinforcement layer auxiliary materials, reinforcement ribs, connecting discs and vacuum auxiliary materials are laid according to the requirements, epoxy resin is vacuum poured, and after cooling and forming, a back plate integrally formed with the reinforcing ribs and connecting discs is obtained, and then in the The reflective glass is glued on the back plate to obtain a composite material back plate. Compared with the prior art, the composite material backboard prepared by the invention has uniform structure, high strength, low cost, long service life, simple preparation process and convenient installation of the backboard.