Abstract: The extraction process of polysaccharide from Sophora japonica was optimized, and the single polysaccharide component (named SJP) was purified, then the structure characterization and antioxidant activity of SJP were investigated. Using crude polysaccharide yield as evaluation index, single factor combined with response surface method were used to explore the optimal conditions of water extraction of Sophora japonica polysaccharide. The crude polysaccharide extracted under the optimal conditions was further purified by deproteinization, dialysis and Sephadex G-200 gel column chromatography. The molecular weight of SJP was determined by high performance liquid chromatography (HPLC), and its chemical composition was analyzed. Congo red experiment and X-ray diffraction (XRD) analysis was applied to characterize the SJP structure. With the free radical scavenging rate and total reducing power as indicators, the antioxidant activity of SJP was investigated. The results showed that the optimal extraction conditions were solid-liquid ratio of 1:20 g/mL, extraction time of 120 min and alcohol precipitation concentration of 60%. Under these conditions, the polysaccharide yield was verified with 3.94%. HPLC results showed that SJP was a homogeneous polysaccharide with the molecular weight of 2.32×10⁶ Da. The contents of total sugar, protein and uronic acid in SJP were 93.47%±0.83%, 1.62%±0.13% and 7.13%±0.51%, respectively. Congo red experiment showed that SJP did not contain triple helix structure, and XRD results indicated that SJP had low crystallinity and amorphous structure. The IC₅₀ values of SJP for scavenging DPPH, OH and ABTS⁺ radicals were 1.09, 4.31 and 1.39 mg/mL, respectively, and the total reducing power was 0.57 at concentration of 5 mg/mL, indicating that SJP had certain antioxidant activity. In conclusion, the process of extracting and purifying polysaccharide from Sophora japonica in this study was feasible, and the obtained polysaccharide had high purity and exhibited good antioxidant activity.