The invention discloses a preparation method of lycium barbarum polysaccharides and belongs to the technical field of food processing. The preparation method of lycium barbarum polysaccharides comprises steps as follows: sieving, impurity removal, drying, crushing, separation, supercritical CO2 extraction, soaking, enzymolysis, centrifugation, desalination, four-level ultrafiltration, reverse osmosis concentration, freeze sublimation drying, low-temperature crushing, packaging and obtaining of a finished product, namely, the lycium barbarum polysaccharides. The preparation method of the lycium barbarum polysaccharides has the characteristics of simple process, high operability and the like. The lycium barbarum polysaccharides are extracted in an ultrafiltration and reverse osmosis concentration combined manner, the rejection rate of the lycium barbarum polysaccharides can reach 94.3%, and the largest content of the lycium barbarum polysaccharides reaches 65% or above. According to the lycium barbarum polysaccharides prepared with the method, the anti-oxidation index of the lycium barbarum polysaccharides is tested to be larger than or equal to 150,000, and the highest index can reach 163,000; 100 times or more of experiment researches prove that the technical data tend to be stable, the product quality is reliable, and industrial and large-scale production is easy to realize.