Objective： To study the antioxidant activity of different polysaccharide components from Inonotus obliquus in vitro and in vivo. Methods： Five water-soluble polysaccharides （IOP40， IOP50， IOP60， IOP70 and IOP80） were isolated from the water extracts of Inonotus obliquus by different volume fraction of alcohol precipitation. DPPH free radicals， hydroxide free radicals， and ABTS+ free radical scavenging experiments were used to evaluate the antioxidant activity of polysaccharide from the Inonotus obliquus in vitro. T-AOC， SOD， MDA， CAT， GSH-PX in liver and T-AOC， GSH-PX， MDA， LDH in serum of mice in different treatment groups were used to evaluate the effect of Inonotus obliquus polysaccharide on antioxidant activity in vivo. Results： In vitro experiments showed that ·OH， DPPH· and ABTS+· scavenging rate of the water extracts and five different polysaccharide components of Inonotus obliquus were increased with the increase of mass concentration within the mass concentration range. Among which IOP40 and IOP50 had better antioxidant effect. The IC50 values of IOP40 to three kinds of free radicals were （0.916 ± 0.08）， （0.660 ± 0.14） mg/mL and （0.091 ± 0.03） mg/mL. The IC50 values of IOP50 were （0.837 ± 0.07）， （0.837 ± 0.12） mg/mL and （0.108 ± 0.005） mg/mL. In vivo experiments showed that IOP50 could significantly increase the levels of antioxidant-related enzymes and metabolites in aging mice compared with IOP40 and water extracts. Conclusion： The polysaccharide fraction isolated from Inonotus obliquus had certain antioxidant activity， especially IOP50 could significantly alleviate oxidative stress damage and delay aging of the body.