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.