Abstract: Objective:Isolation,purification and acetylation of Pleurotus eryngii Quel polysaccharide was conducted to obtain acetylated derivatives with different substitutive degrees and compare their antioxidant activities. Method:The crude polysaccharide was obtained through hot water extraction and ethanol precipitation,then it was purified by DEAE-Sepharose Fast Flow anion exchange column and Sephacryl-S400 molecular sieve column to gain homogenital components. The homogenital components was then modified by adding three different volumes of acetic anhydride to get different degrees of substitution and evaluated using in-vitro antioxidant-activity models. Results:After acetylation modification,three acetylated derivatives were obtained which were named Ac-PEP-I-1-a,Ac-PEP-I-1-b and Ac-PEP-I-1-c with different degrees of substitution 0.132,0.406 and 0.537.Antioxidant research found that both acetylated derivatives and unmodified polysaccharide showed free radical scavenging activities with concentration and activity positively correlated. Conclusion:Acetylation enhanced the scavenging activity of hydroxyl and superoxide anion radicals,the higher the substitution,the stronger the activity;conversely,acetylated modification reduced the ability of scavenging DPPH free radical and acetylated derivatives with high the substitution turned out lower activity.