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