A composite membrane, including a base plate; the base plate is used for corresponding to a light source, and an emergent surface of the base plate is provided thereon with multiple dispersive prism areas and multiple diffusion plates, wherein the dispersive prism areas correspond to lamp bead shadow areas, and the diffusion plates correspond to non-lamp bead shadow areas; the dispersive prism areas are provided in areas in which light energy is distributed unevenly, while only the diffusion plates are laid in areas in which light energy is distributed evenly, and the dispersive prisms need not be laid, thereby reducing production costs while improving the phenomenon of a visual effect being uneven. Further provided is a design method for a composite membrane.