Abstract: To study the graded alcohol precipitation components of fucoidan from Undaria pinnatifida and its hypoglycemic activity in vitro, this study used Undaria pinnatifida as raw material, extracted Undaria pinnatifida fucoidan gum through CaCl₂ solution, and precipitated it step by step to obtain four sets of polysaccharide samples (UPP-45, UPP-55, UPP-65, and UPP-75). Determined their molecular weight, total sugar content, uronic acid content, protein content, sulfate content and monosaccharide composition and analyzed by ultraviolet spectrum (UV), Fourier transform infrared spectra(FT-IR) and scanning electron microscopy(SEM). The inhibition of polysaccharides on α-amylase and α-glucosidase was determined to explore their hypoglycemic activity in vitro. The results showed that the higher the concentration of alcohol precipitation, the higher the proportion of small molecular weight polysaccharide. The highest total sugar content of UPP-65 was 84.38%, and the protein of the crude polysaccharide and sulfate contents of UPP-45 were significantly different from those of other components (P<0.05). All the four polysaccharide components were acidic polysaccharides. The monosaccharide composition analysis showed that the monosaccharide composition and mole percent of polysaccharide components were different. The UV results showed that the content of nucleic acid of polysaccharide components was different. The FT-IR results showed that four components had different levels of absorption peaks of functional groups and glycosidic bonds. The SEM results showed that the microscopic morphology of polysaccharide components was different. The inhibition of α-amylase was not dose-dependent, and the inhibitory rate of UPP-65 was the highest with IC₅₀ value of 0.008 mg/mL. The inhibition of α-glucosidase showed a dose-effect relationship, and UPP-65 had the lowest IC₅₀ at 2.346 mg/mL. In summary, the fractional alcohol precipitation of Undaria pinnatifida polysaccharides technology was feasible. All the four polysaccharide components had certain hypoglycemic activity in vitro, and UPP-65 had the better effect.