The composition of organic acids in summer lees of fen liquor were analyzed via high performance liquid chromatography in this paper.Acidogenic bacteria were isolated from summer normal fermented grains and high acid fermented grains by flat crossed method.The influence factors of lees acidity were studied and fen liquor brewing process was optimized.Conclusion was: acetic acid and lactic acid were primarily organic acids in fermented grains;23 strains acetic acid bacteria and 5 strains lactic acid bacteria were isolated from normal fermented grains;23 strains acetic acid bacteria and 29 strains lactic acid bacteria were isolated from high acid fermented grains.Environmental temperature,moisture of fermented grains and amount of each urn packed greatly influenced the acidity of fermented grains.On this basis,suitable acid brewing conditions were obtained using the response surface method: with the environment temperature of 20 ℃,moisture of fermented grains of 51% and amount of each urn packed of 0.83 kg.The total acidity of fermented grains was 2.12(0.1 mol NaOH mL/g fermented grains),which was consistent with the model prediction of 2.07.The results provided scientific basis to solve the problem of high acid lees of fen liquor in summer.