Abstract: The antioxidant activities of ethanolic extract from Pleurotus eryngii were evaluated by various methods in vitro. Some bioactive components of ethanolic extract were analysed by high performance liquid chromatography mass spectrometry(LC-MS). In the ethanol extract of Pleurotus eryngii,the protein content was 2.41 mg/g(DW),the polysaccharide content was 25.46 mg/g(DW),polyphenol content was 12.03 mg/g( DW) and the flavonoids content was 1.53 mg/g( DW). In antioxidant assays in vitro, ethanolic extract of Pleurotus eryngii was found to have strong reducing power,superoxide radical,hydroxyl radical and 2,2-diphenyl-1-picrylhydrazyl radical scavenging activity. The results also showed that with the concentration of ethanol extract of Pleurotus eryngii increased,the antioxidant activity also increased. Free radical scavenging capacity of ethanolic extracts from Pleurotus eryngii was:·OH>DPPH·>O2-,and the ethanol extract of Pleurotus eryngii had a certain ability of reducing ability and anti lipid peroxidation. The main phenolic compounds in ethanolic extract analyzed by ultra-high performance liquid chromatography tandem mass spectrometry were quinic acid,gallic acid,catechin,and p-coumaric acid and cinnamic acid.