Abstract: Acetylated modification was applied to modify the structure of polysaccharide from Inonotus obliquus,and the antioxidant activity of polysaccharides were evaluated,which was expected to provide the new ideas and directions for the structural modification of Inonotus obliquus. Inonotus obliquus was used as raw materials. Polysaccharides from Inonotus obliquus were obtained by water extraction and alcohol precipitation. Using inonotus obliquus as raw material, the polysaccharide of inonotus obliquus was extracted by water extraction method,and was purified after AB-8 macroporous adsorption resin. The polysaccharides of inonotus obliquus were prepared by acetic anhydride with different volume in sodium hydroxide aqueous solution,and the substitution degree was determined by hydroxylamine hydrochloride colorimetric method. On this basis,the antioxidant activities of three different acetylated degrees of acetylated Inonotus obliquus polysaccharides were studied. Results:After acetylation modification,three acetylated derivatives were obtained with different degrees of substitution 0.2810,0.3136,0.4072.With the increase of the volume of acetylation reagent,the degree of substitution of polysaccharides increased gradually. With the increase of substitution degree,the antioxidant activity of polysaccharides gradually enhanced. The maximum scavenging rates of DPPH were 71.8%,74.9% and 78.5% for three acetylated products with different degrees of substitution,respectively. The maximum scavenging rates of O₂^- was 71.8%,74.9% and 78.5%,respectively. The maximum scavenging rates for OH^- were 74.5%,79.9% and 81.1%,respectively. In conclusion,in vitro antioxidant activity of polysaccharides from Inonotus obliquus is significantly enhanced by acetylation. In addition,the strength of antioxidant activity was related to the degree of acetyl substitution.