Analytical methods based on mass balance were adopted in the experiment to trace the carbon source distributed and utilized in a pilot scale system. The key parameters of carbon source distribution were investigated and the optimum operation strategy for urban sewage treatment plants was built. In the conditons that the influent concentration of the COD, NH+4-N, TN and TP were 129 mg/L，25.6 mg/L，31.5 mg/L and 3.38 mg/L, respectively, and the C/N and C/P value were 4.3 and 39.5, inverted A2/O process was recommended in winter period, enhanced A2/O process was recommended in spring period, pre-anoxic inverted A2/O process was recommended in summer period and alternating oxygen concentration inverted A2/O process was appropriate in autumn period. In this case, the ratio of effluent COD to influnent COD was 26.1%～29.4%, the ratio of assimilating COD was 27.5%～36.2%, the ratio of directly oxidating COD was 4%～22.2%, the ratio of denitrificating COD was 14.8%～33.6%, and the ratio of COD utilizing for removal was 3.05%～6.9%. The effluent of the pilot scale reactor could reach the standard of grade B, classⅠin Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB 18918-2002) apart from the concentration of phosphorus. The rationalization of carbon source utilization should be an important rule for the design and adjustment of wastewater treatment plants.