The invention discloses a preparation method of an all-solid-state environment-friendly biopolymer electrolyte film, and relates to a preparation method for a lithium ion battery electrolyte thin film. The problems of low electric conductivity, high cost and poor environmental protection of a lithium ion battery electrolyte thin film prepared by conventional methods are solved. The method comprises the following steps: firstly, preparation of modified starch: taking a proper amount of starch, phthalic anhydride, pyridine and a solvent, under water bath conditions, blending and stirring until the solution becomes a uniform paste substance, then modifying starch with an isopropanol precipitate, next drying in a vacuum drying box, and finally grinding the obtained substance into a powder; andsecondly, preparation of the all-solid-state environment-friendly biopolymer electrolyte film: weighing cellulose, modified starch, a lithium salt and a solvent, blending and stirring, and then drying in the vacuum drying box to form the film. The prepared all-solid-state environment-friendly biopolymer electrolyte film has the advantages of good biodegradability, simple experimental process, wide sources of experimental raw materials, low cost and wide application range.