Abstract: In this paper，semi-continuous flow experiments on an aerobic digestion in middle temperature for excess sludge from wastewater treatment plant of Jinzhou petrochemical plant of china petroleum is adopted.Fluctuation conditions of pH,VFA，alkalinity，COD of solid-liquid and degradation mechanism of COD、VSS are all examined in the process of quantified input raw sludge daily and sludge retention time respectively.It can be learnt by the experiment that sludge retention time has an effect on anaerobic digestion mechanism experiment and reaction outcome.The result shows that: removal ratio of COD is 70.3%，removal ratio of VSS is 55.6%，and gas
production is 0.33m3 per kg COD sludge mixture.Mixed anaerobic digestion treatment of excess sludge achieves reduction of volume and resource recovery of excess sludge.However，both removal ratio of organic materials and sludge decomposition rate are much lower than those in the process of single digestion.So，more pilot study is needed，and cultivation and acclimatization of spawn preceded，so that dominant spawn can be used to improve treatment efficiency and gas rate of excess sludge.

Key words: excess sludge, resource the mankind, anaerobic digestion, biogas generation, digestibility