Abstract: Objective:To identify the purity and analyze the monosaccharide composition of a polysaccharide isolated and purified from Agaricus brasiliensis. Methods:The polysaccharide content of Agaricus brasiliensis polysaccharide was determined by phenol-sulfuric acid method,and its purity was identified by Sephadex Filtra,High-Performance Liquid Chromatography-Evaporative Light Scattering Detection(HPLC-ELSD)method and UV-Visible Absorption Spectroscopy. High Performance Liquid Chromatography(HPLC)-Column Derivatization method was used to determine the monosaccharide composition of the polysaccharide. Results:The purity of the obtained polysaccharide was high:its polysaccharide content was 99.50%±2.5%(W/W,calculated as glucose),and it does not contain nucleic acid,protein and other substances. The polysaccharide was mainly composed of mannose,glucose and galactose,which accounts for 0.2%,99.2% and 0.6%,respectively. Conclusion:The obtained Agaricus brasiliensis polysaccharide was a homogeneous polysaccharide with high purity and more glucose content.