Objective To improve the biological activity and application range of Porphyridium aerugineum exopolysaccharide (EPS).Methods EPS was modified using the concentrated sulfuric acid method, with the degree of sulfation as the indicator. The preparation process of sulfated P. aerugineum exopolysaccharide (S-EPS) was optimized using response surface methodology. The physicochemical properties of S-EPS were also determined.Results The optimal conditions were a reaction time of 6 h, a reaction temperature of 26 ℃, and a mass ratio of ammonium sulfate to EPS of 4∶5. Under these conditions, the degree of substitution was 0.807 5±0.008 7. Infrared analysis confirmed that EPS was successfully sulfated. Thermogravimetric analysis showed that the thermal stability of S-EPS was superior to that of EPS. Particle size and Zeta potential analysis indicated that samples with a higher degree of substitution were more stable, with higher —SO₃H group content, larger absolute potential, and less aggregation. The results of the antioxidant activity experiment showed that the DPPH· clearance rate of S-EPS was 92.17%, significantly higher than EPS.Conclusion Sulfation modification improved the antioxidant capacity of EPS, indicating that sulfation is a feasible method for enhancing polysaccharides.