The invention discloses a preparation method of sargassum oligosaccharide. The preparation method comprises the steps that fine polysaccharide obtained through degreasing and deproteinizing of sargassum is taken as a raw material, ultrasonic treatment is utilized, alginate lyase, mannase, xylanase and pectinase are sequentially added to perform enzymolysis on the polysaccharide, the polysaccharide which is not degraded fully is removed through an ethyl alcohol sedimentation method, supernate is centrifuged and screened by a molecular sieve to obtain retained matter, freeze drying is performed, and the sargassum oligosaccharide is prepared. The prepared sargassum oligosaccharide has the high inhibitory activity on alpha-glucosidase, the IC50 value is 4.82 mg/mL, the dose dependency is presented, and meanwhile the obvious promoting effect on glucose consumption of insulin-resistant HepG2 cells is achieved. According to the preparation method, the non-specific commercial enzymes are adopted, the process route is simple and reasonable, the method is suitable for industrialization, the preparation amount of the hypoglycemic active oligosaccharide is increased, and meanwhile the enzyme consumption and production cost are reduced; the method is an effective method for preparing the sargassum oligosaccharide and can be applied to hypoglycemic drugs, health care products and food.