The invention relates to the technical field of optical thin film, in particular to a diffusion membrane capable of enlarging the viewing angle. The diffusion membrane is used for further enlarging the viewing angle of a backlight module. The diffusion membrane comprises a transparent base material, a diffusion coating and a protection coating, wherein the diffusion coating and the protection coating are arranged on the two sides of the transparent base material respectively, the diffusion coating comprises, by weight, 100 parts of adhesives and 75-120 parts of diffusion particles, the diffusion particles are adhered to the upper surface of the transparent base material through the adhesives, the particle diameters of the diffusion particles are the same, or the particle diameter variable coefficient of the diffusion particles is smaller than or equal to 15%, and the particle diameters of the diffusion particles are 8-35 microns. The diffusion membrane has the function of improving luminance and the function of enabling light to be even, and can effectively enlarge the viewing angle of the backlight module, and thus the viewing angle of a liquid crystal displayer is enlarge, and the watching effect of the liquid crystal displayer can be better.