Abstract: In step-feed A/O biological nitrogen removal system,the optimal influent flow rate distribution can make full use of the carbon source in influent,and the appropriate anoxic/oxic volume ratio can save the operation cost and resist the influent disturbance effectively.Based on the characteristics of configuration and operation of step-feed process,this paper analysed the influent flow rate and volume distribution theoretically and discussed the optimization of operation.Regarding to the influent flow rate distribution,the influent C/N ratio was employed as a control parameter,and the distribution method under three different conditions when the amount of carbon saurce was sufficient,exactly appropriate and insufficient respectively,was discussed. Then the methods were given to optimize the operation.In addition,the effect of volume distribution on the capacity of nitrification and the utilization of COD in influent was also analyzed and the optimization of system under the different loads was discussed.