An acidic extracellular polysaccharide (EPS) was gained from Stropharia rugosoannulata fermentation broth by the alcohol precipitate method. Sulfated derivative of EPS (S-EPS) was gained by chlorosulfonic acid-pyridine method and the conditions were as follows: pyridine as the reaction solvent, temperature 80 °C, and reaction time 90 min. The chemical structure and chain conformation of EPS and S-EPS were characterized by FT-IR, NMR and size exclusion chromatography (SEC) coupled with multi-angle laser light scattering (MALLS) detection.. FT-IR spectra indicated the sulfated group was linked to the polymer. The EPS mainly consists of mannose, glucose and glucuronic acid, and the glycosidic bond type is mainly (1→4)-linked and (1→6)-linked. C6 and C2 were substituted by sulfate groups. The EPS was degraded into smaller molecular weight (Mw) polysaccharides with the larger molecular size (Rg) after sulfation and S-EPS has better hydrophilicity. Moreover, the sulfated derivative S-EPS was found to have a better scavenging ability on hydroxyl radicals than EPS. It proved that the chemical modification of S. rugosoannulata polysaccharides by sulfation effectively enhanced their antioxidant activity.