Abstract: In this study, the preparation process of selenized Bletilla striata polysaccharides (BSP-Se) was optimized, to explore whether higher antioxidant activity preparation could be obtained. BSP-Se was prepared by using Bletilla striata polysaccharide (BSP) as the raw material, Na₂SeO₃ as the selenifying agent, and acetic acid as the catalyst. Taking selenium content as an indicator, the preparation process was optimized using single factor experiments and response surface methodology. Fourier-transform infrared spectroscopy was used to determine whether BSP-Se was successfully prepared, and the antioxidant activity of BSP-Se in vitro was evaluated by measuring the scavenging ability of DPPH, ABTS⁺ and ·OH free radicals. The results showed that the selenium content of BSP-Se prepared under the optimal process conditions of Na₂SeO₃ to BSP mass ratio of 1.5, acetic acid to BSP dosage ratio of 3, reaction time of 6 hours, and reaction temperature of 80 ℃ was the highest, reaching 7.831 mg/g. FT-IR analysis showed that selenium binds to BSP through Se-O-C and O-Se-O, indicating that the polysaccharides have been successfully selenified. The analysis of free radical scavenging ability showed that the IC₅₀ values of BSP-Se for DPPH, ABTS⁺, and ·OH free radicals were 2.875, 4.431 and 0.314 mg/mL, respectively, which were significantly lower than the IC₅₀ values of 5.828, 8.475 and 0.833 mg/mL of BSP (P<0.05), indicating that selenization modification improved the antioxidant capacity of BSP. Therefore, the antioxidant activity of BSP can be improved by selenization modification, which provides a theoretical basis for further research on the application of selenium polysaccharides as a new antioxidant in the fields of foods and medicine, health care products and daily chemical.