Abstract: In this study, the physicochemical properties and antioxidant activity of alkali-extracted soluble dietary fiber (SDF) extracted from Sporisorium reilianum were investigated. According to the single-factor experiment results, the extraction process of SDF was optimized by response surface methodology. Response surface analysis results indicated that the maximum extraction yield of SDF reached up to 20.21% under the optimized conditions with the solid-to-liquid of 1:21.40 g/mL, alkali concentration of 2.11%, alkalinization time of 90.71 min and extraction temperature of 59.30 ℃. Meanwhile, the results showed that the water holding capacity, oil holding capacity and swelling capacity of SDF were 3.48±0.05 g/g, 1.50±0.07 g/g and 13.22±0.03 mL/g, respectively. In addition, the experimental results also showed that SDF had high antioxidant activity and the scavenging rate of free radicals was positively correlated with the concentration of SDF. The concentration of 3.5 mg/mL scavenged 62.02% ·OH, 56.98% DPPH·, and 61.03% O₂^-·, respectively. These results indicated that Sporisorium reilianum was a potential source of natural dietary fiber and a potential functional food ingredient.