Optimal selection of interception ratio of combined sewer systems is a problem of comparison in technical and economical aspects. Using the simulation result of SWMM, the relationship between pollutant removal efficiency and interception ratio can be quantified, so the technical index of interception ratio selection can be obtained. Regarding of pollutant removal efficiency and construction cost as functions of interception ratio, two objective functions of optimization model can be obtained. By solving this optimization model, the balance between technical and economical aspects can be realized at the result interception ratio. Tested by the case sewer system, an optimal result can be obtained by this optimization model and it’s solving methodology.