The invention belongs to the fields of health foods and medicines and discloses a Huidouba polysaccharide and a preparation method thereof. The preparation method comprises the steps of extracting Huidouba as a raw material with hot water, carrying out ethanol precipitation to obtain rough Huidouba polysaccharide, carrying out activated carbon decoloration, removing protein by virtue of a Sevage reagent, carrying out separation and purification by virtue of a ultrafiltration technique, and screening to obtain a Huidouba polysaccharide 2 with relatively low molecular weight, wherein the peak molecular weight Mp is 6277Da, the weight-average molecular weight Mw is 9703Da, and the number-average molecular weight Mn is 7144Da. By virtue of the preparation method, the Huidouba polysaccharide 2 can be efficiently prepared through separation, is capable of remarkably reducing blood glucose of type 2 diabetes KKAy mice, glycosylating hemoglobin, improving glucose tolerance and glucolipid metabolism, has an in-vitro antioxidation effect and is applicable to the preparation of health Huidouba polysaccharide foods and medicines with the hypoglycemic effect.