Abstract: The polysaccharide was obtained with the water extraction and alcohol precipitation method. The total carbohydrate content and monosaccharide composition were measured by phenol-sulfuric acid method and1- phenyl- 3- methyl- 5- pyrazolone pre- column derivatization high performance liquid chromatography( HPLC) method, resepectively. Moreover, the total reducing power and reducing power of Pleurotus eryngii polysaccharides on DPPH and hydroxyl radicals and superoxide anion free radical in vitro was measured. The total carbohydrate content of Pleurotus eryngii polysaccharide was 62.9%,which was composed of D-mannose,D-ribose,D-rhamnose,D-glucose,D-glucuronic acid,D-xylose,D-galactose and D-fucose with a relative molar percentage of 9.8%,1.6%,0.15%,0.8%,62.8%,0.05%,24.4%,0.4%. The results showed that:Pleurotus eryngii polysaccharide was a typical heteropolysaccharide and had strong antioxidant activity.