The invention discloses a method for extracting an alpha-amylase and alpha-glucosidase inhibitor from bangia fusco-purpurea,which comprises the following steps of 1, extraction: cleaning, drying and smashing fresh bangia fusco-purpurea to obtain bangia fusco-purpurea powder, adding distilled water, extracting and concentrating to obtain a concentrated solution; 2, alcohol precipitation: dropwise adding ethyl alcohol, centrifuging, taking sediments, performing freeze drying to prepare crude polysaccharide; 3, gel chromatography: dissolving the crude polysaccharide into distilled water, sample loading, eluting, tracing and checking, collecting eluting peaks, performing dialysis desalting, andperformingfreeze drying to obtain purified polysaccharide, so that the alpha-amylase and alpha-glucosidase inhibitor is prepared. The method provided by the invention has the advantages that the preparation technology is simple, the cost is low, and the extracted alpha-amylase and alpha-glucosidase inhibitor has the activity for inhibiting blood glucose elevation.