In order to study the structural characteristics of walnut shell polysaccharide and the antioxidant activities of its derivatives in vitro,walnut shell polysaccharide were prepared by water extraction and alcohol precipitation,and two derivatives of acetylated and phosphorylated walnut shell polysaccharide were prepared by chemical synthesis.The molecular weight,monosaccharide components,molecular structure characteristics and antioxidant activities of its derivatives were studied comprehensively and deeply,and a series of meaningful data information for the study of walnut shell polysaccharide and its derivatives were obtained.The results showed that the molecular weight of the homogeneous polysaccharide was 9.45×10⁴ Da and the total sugar content was 98.51% after DEAE-52 and Sephadex G-100 separation.The results of monosaccharide determination showed that walnut shell polysaccharide was an acidic heteropolysaccharide containing rhamnose,arabinose,galactose,glucose,xylose and galacturonic acid.The molar ratio of monosaccharide components was 6.7∶16.5∶28.3∶11.2∶12.5∶24.8.The results of antioxidant activity in vitro showed that walnut shell polysaccharide and its derivatives showed a good biological activity,especially the phosphorylated walnut shell polysaccharide had the best activity.The scavenging rates of superoxide anion,hydroxyl free radical and DPPH free radical were 92.0%,96.5% and 92.3% respectively when the mass concentration of walnut shell polysaccharide was 3.2 mg/mL.