The invention relates to a method for preparing a cotton cellulose nano fibril film, which comprises the technical steps of 1, chemical pretreatment; 2, mechanical separation; and 3, film preparation. The method has the advantages that the film is prepared by combining acid-base pretreating and grinding wood meal, the method is mainly used for removing lignin in the wood meal under the acidic condition of sodium chlorite, the diluted potassium hydroxide is then used for removing the hemicellulose in the wood meal, the residue is basically cellulose, and finally, the diluted hydrochloric acid is used for splitting the cellulose. The internal hydrogen-bonding acting force of the fibril is reduced through the splitting treatment, so the biomass cellulose nano fibril with high length-diameter ratio is obtained. The hydrochloric acid treatment is utilized to remove the mineral substance in the fiber, the alkaline insoluble hemicellulose of the purified cellulose is influenced, and the effect of purifying the cellulosic fiber is realized. Chemical constituents are detected through Fourier transform infrared. The grinding is simple and easy to operate, and the film can be produced in mass. The elastic modulus is 3463.23 MPa, and the tensile strength is 87.38 MPa.