Abstract: Selenium polysaccharide has multiple functions of anti-metal poisoning,anti-oxidation,anti-virus etc. Its activity is closely related to its structure. In this paper,the Potentilla anserina L. polysaccharide extracted from traditional Tibetan medicine potentilla was used as a precursor. Optimizing the preparation process and regulated the conditions by single factor and response surface design to prepare different substitution degrees of Potentilla anserina L. polysaccharides. The size exclusion chromatography-laser light scattering spectroscopy,UV spectrometer,infrared spectrometer,thermogravimetric analyzer,and scanning electron microscopy were used to characterize the structure of the product and evaluated the in vitro antioxidant activity of different substitution degrees of selenium Potentilla anserina L. polysaccharide in the end. The results showed that the optimum reaction time of selenium Potentilla anserina L. polysaccharide was 134.84 min,temperature 78.39 ℃,catalyst 49.37 mg and wight ratio PAP:H₂SeO₃=1:0.81. Under these conditions,the selenium contents of the product was 8518.81 μg/g. There existed Se=O and Se-O bonds in selenium Potentilla anserina L. polysaccharide with five degrees of substitution,meaning the selenization of the Potentilla anserina L.polysaccharide was successful. The higher degree of substitution of selenium Potentilla anserina L. polysaccharide,the smaller the molecular weight,the lower the thermal stability,and the looser the spatial structure. The selenium Potentilla anserina L. polysaccharide had significant scavenging ability to O₂^-·,·OH and DPPH·,and had positive correlation to the substitution degree of selenium.