The invention discloses a compound 2,3,20-trihydroxy-29-noroleanolic acid, as well as a preparation method and application thereof in preparing glycosidase inhibitor medicines. According to the preparation method, a powerful alpha-glycosidase inhibitor is extracted and separated from akebia plants, the plant material is rich in source, the extracting preparation method is easy to operate, and the plant cannot be destroyed and can be utilized for a long time when the plant fruits are adopted for extraction, so that the economical benefit can be increased, and the environment is protected. The monomeric compound is stable and easy to store. Pharmacological experiments indicate that the activity of the alpha-glycosidase inhibitor of the compound 2,3,20-trihydroxy-29-noroleana-12-ene-28-oic acid is stronger than that of the first line diabetes drug acarbose, and the compound is expected to be developed into a medicine for treating II-type diabetes in clinical application, and has good prospect.