The invention belongs to the technical field of optical films, particularly relates to the optical light separation film technology, and particularly relates to a visible light band (0.4-0.7 micron) reflection and long-wave infrared band (7-10 microns) transmittance all-dielectric film color separation element and a design method thereof. Compared with the conventional dielectric-metal-dielectric film system structure, the all-dielectric film material is designed on the ZnS or ZnSe substrate of the color separation element, and light separation is performed according to the mode of visible light reflection and infrared transmittance so that the average light separation efficiency of the visible light can be more than 80%, and the average light separation efficiency of the long-wave infrared light can be more than 96%.