The invention belongs to the technical field of optical films, and specifically relates to an all-dielectric film color separation element reflected in a visible light waveband (0.6-0.9 [mu]m) and a laser waveband (1.06 [mu]m) and transmitted in a middle-infrared waveband (3.0-5.0 [mu]m) and a design method. According to the color separation element and the design method, light splitting is performed according to the methods of visible light and laser reflection and infrared transmission, a silicon material substrate is designed by employing an all-dielectric film material, which is different from a conventional dielectric-metal-dielectric film structure, the average light-splitting efficiency of the visible light and laser wavebands reaches more than 98%, and the average light-splitting efficiency of the middle-infrared waveband reaches more than 97%.