Abstract: Water soluble Angelica sinensis polysaccharide (ASP) extracted from Angelica sinensis was obtained by ultrasonic extraction, and was chemically modified by sulfation, phosphorylation, acetylation and carboxymethylation. To obtain strong Angelica sinensis polysaccharide antioxidant activity, we used the infrared spectrum to characterize structure and study antioxidant activity. Results indicated that chemical modification of ASP had corresponding characteristic absorption peaks, namely the four kinds of chemical modification were successful. Different results were obtained by measuring antioxidant activity in vitro and the scavenging free radical capacity of the five kinds of polysaccharide. Total reducing power of the four modified polysaccharides was weaker than that of the unmodified ASP, and no obvious enhancement in scavenging hydroxyl radicals was observed in the modified polysaccharides, though the in vitro capacity of DPPH^· radical scavenging and inhibition of Fe²⁺ induced lipid peroxidation were enhanced. P-ASP was the strongest at scavenging the superoxide anion radical, Ac-ASP was the strongest at inhibiting Fe²⁺ induced lipid peroxidation, and they all showed a certain dose-effect relationship. The results of this study provide a scientific basis for the further study and development of Angelica sinensis polysaccharides.