The invention discloses a preparation method of a soybean protein/sodium alginate cross-linked membrane for food packaging and belongs to the technical field of preparation of biological materials and food packaging materials. The preparation method comprises the following steps of respectively preparing a soybean protein solution and a sodium alginate solution with distilled water, stirring according to mass-to-volume ratios of 4:6, 5:5 and 6:4 and uniformly mixing; adding a crosslinking agent, carrying out cross-linking reaction and dialyzing to remove the crosslinking agent; and then horizontally placing the membrane-forming solution in a glass flat plate and drying for 12-16 hours at 40 DEG C to obtain the cross-linked membrane at different ratios. The cross-linked membrane is firstly balanced for 48 hours under the conditions that the temperature is 40 DEG C and the relative humidity is about 50% and then characterized. The elongation at break of the cross-linked membrane is far greater than that of a soybean protein/sodium alginate blended membrane and single sodium alginate. The cross-linked membrane has the advantages of good water resistance and oxygen resistance, good biocompatibility and biodegradability, no toxicity, no odor, no irritation and the like and can be widely applied to the field of food packaging.