Medicine for preventing and treating alzheimer's disease and preparative method thereof

The present invention discloses a medicine for preventing and treating Alzheimer's disease, wherein the effective constituent is quercetin-3-O-β-D-xylopyranosyl-(l→2)-β-D-galactopyranoside (RNFG) extracted from Panax Notoginseng RNFG has estrogenic activity, antioxidant activity and protecting effect on nerve by preventing the assembly and toxicity of Aβ. RNFG also can decrease memory injury induced by scopolamine, and can be used in the manufacture of medicaments and health care products for preventing and treating Alzheimer's disease.