Abstract: OBJECTIVE To optimize the extraction process of polysaccharides in fresh Codonopsis pilosula fermented by yeast and evaluate its immune activity in vitro.METHODS Using yeast-fermented fresh Codonopsis pilosula as raw material,the polysaccharides in yeast-fermented fresh Codonopsis pilosula were obtained by papain-assisted extraction and ethanol precipitation.With extraction temperature,time and enzyme content as the influencing factors,polysaccharide yield as the index,and water extraction as the control,single factor experiment combined with response surface design was carried out to optimize the process.The relative molecular weight of polysaccharides was determined by high performance gel permeation chromatography,and the proliferation activity of polysaccharides on spleen lymphocytes and RAW264.7 cells in vitro was studied by MTT assay. RESULTS The optimal extraction conditions of polysaccharides from fresh Codonopsis pilosula by yeast fermentation were as follows:extraction temperature 63℃,extraction time 6 h,enzyme content 4%.The extraction rate of polysaccharide was 17.46%,which was higher than 12.69% by water extraction.And the uronic acid content increased.The relative molecular weight of polysaccharides obtained by enzyme extraction and water extraction had no obvious change.When the polysaccharide concentration was 0.5,1.0 mg·mL^-1,the proliferation activity of spleen lymphocytes and RAW264.7 cells treated with enzyme-extracted polysaccharide increased compared with that treated with water extraction.CONCLUSION In this study,the optimized extraction process of polysaccharides from fresh Codonopsis pilosula by yeast fermentation is feasible.The extraction rate of polysaccharides is high and the immune activity is significantly enhanced.