Abstract: Cellulase was added to the hot water extraction of polysaccharides from Chlorella pyrenoidosa in an attempt to facilitate the process. The single factor test and the response surface methodology were used to optimize the conditions. It was found that whenthe substrate:liquid ratio of 1:30(gmL-1) with 2%(w/w) of cellulase addition was extracted at 93℃ for 2 h, an expected 6.13% yield on polysaccharidescould be obtained (the actual yield was 5.870.24%). Subsequently, the polysaccharides obtained was tested for its scavenging activities on DPPH,O2-, and hydroxyl free radicals.The results confirmed itsantioxidant abilities agains O2- and hydroxyl free radicals.