The invention discloses a composite film capable of reducing blue light hazard. The composite film comprises an upper base layer, a lower base layer, and a microcrystal structure layer which is arranged between the upper base layer and the lower base layer and is composed of photonic crystals, and the microcrystal structure layer can transmit light rays with the wavelength being 450 nm or above and can partially or completely reflect the light rays with the wavelength being 450 nm or below. The invention also discloses a preparation process of the composite film, and the preparation process comprises the following steps: preparing the microcrystal structure layer: preparing the upper base layer and the lower base layer respectively; and laminating the upper base layer, the microcrystal structure layer and the lower base layer into a whole, wherein the microcrystal structure layer is located between the upper base layer and the lower base layer, and an intensifying structure is locatedon the surface layer. The invention further discloses a backlight module which comprises the composite film. The composite film disclosed by the invention is simple in structure, can accurately blockblue light with the wave band of 450 nm or below, and cannot cause color cast at the same time.