Thickness of DCG film is one factor inf1uencing the technical targets of holographic display screen. The change of the thickness of holographic film and the uneven grating structure are analyzed. With the help of Multilayer Optical Theory. its theoretical design is conducted. And the assumption is raised that optical thickness interval in the film is uniform. The computer-slmulation glves us the result of theoretical calculation , which is compared with that of the experiment. In this way we have proved that the assumption of optical thickness is correct.