The velocity and sludge distribution in the aerobic tank of step-feed A/O process were simulated by the two-phase flow model. The aeration process was simulated by the discrete phase model and the influence of the aeration on the sludge distribution was studied. The space distribution of the wastewater-sludge-bubble in the aerobic tank was determined. The results showed that the sludge age is hard to be control led as the eddies in the aerobic tank have a strong influence on the sludge retention time. The impact of the aeration on the mean sludge concentration is neglectable. The uniformity of sludge distribution along the vertical direction and the sludge sedimentation on the bottom layer are improved by the process of aeration. The results have great theoretical significance and guidance for the optimal design of the aerobic tank of step-feed A/O process.