In the invention, polysaccharides are extracted from wolfberry black fruit, and the content of polysaccharides is determined by using anthrone-ethyl acetate-concentrated sulfuric acid solution. Crushing, soaking, heating, filtering, suction filtration and other treatments are performed on the fruit of Lycium barbarum to obtain a solution containing Lycium barbarum polysaccharide. The extracted Lycium barbarum polysaccharides were extracted with water, alcohol-precipitated, and suction-filtered, and the filter cake was dried in an oven. The polysaccharide powder was taken separately, dissolved after adding DMSO, stirred for 12 hours under a magnetic stirrer, added distilled water, and suction-filtered. The precipitate is washed successively with absolute ethanol, acetone and ether, and dried to obtain the refined Lycium ruthenicum polysaccharide; after the obtained refined Lycium ruthenicum polysaccharide is decolorized by macroporous resin and deproteinized by Sevage method, the black fruit Lycium barbarum polysaccharide. Refining with DMSO greatly increases the yield of polysaccharides. When the Lycium barbarum polysaccharides are applied to K562 cells, they can promote cell apoptosis by changing the expression of apoptotic proteins in the mitochondrial pathway, and have anti-leukemic tumor effects.