The invention discloses a preparation method of a straw fiber composite film. The preparation method comprises the following steps: (1) crushing straw, dispersing the straw in water, performing oxidation treatment in the presence of TEMPO (2,2,6,6-tetramethyl-1-piperidine-N-oxo) and pypocholoride, and drying; (2) mixing the dried oxidized straw, polyacrylate and an organic solvent, and softening to obtain a straw mixture; and (3) mixing the straw mixture, polyethylene, citric acid, sodium dihydrogen phosphate, polyvinylidene chloride, hexabromo-benzene and stearic acid, pre-compressing, hot-pressing and sanding, thereby obtaining the straw fiber composite film. The composite film has excellent tensile strength and good breathability and is very applicable to preparation of packaging materials of fruits and vegetables.