The invention provides a preparation method of an environmental-friendly breathable composite film. The preparation method comprises the following steps: pre-soaking and defibering kraft hardwood pulpdry pulp board into fiber slurry suspension, adding bamboo pulp, 2,2,6,6-tetramethyl-piperidin-1-oxyl, sodium bromide and sodium hypochlorite solution, dropping a NaOH solution, and maintaining the pH value of the reaction system; adding ethanol to terminate the reaction; washing the pulp; grinding to obtain pulp A; taking an aqueous solution of ethanol, regulating the pH, adding 3-aminopropyltriethoxysilane for stirring; adding the pulp A and wula sedge xylogen, stirring to obtain pulp B, and performing suction filtration on the pulp B with absolute ethyl alcohol so as to obtain pulp C; mixing, stirring and dissolving polylactic acid and dichloromethane, adding the pulp C, waxy modified corn starch and pea starch solution and sorbitan trioleate, and stirring to obtain a mixed solution; casting the mixed solution, and drying into a film, thereby obtaining the product. The film prepared by the method has excellent mechanical property and excellent breathability, the raw materials are green, non-toxic and reproducible, and sustainable development is well realized.