In allusion to round crosssection concrete sewage pipe, the mathematical model of sewage COD was built based on the one-dimensional diffusing equation. And solution of the model was performed with the finite difference method and program of Visual Basic and Excel. The results show that the model predicts the variations of sewage COD concentrations in the process of sewage transmitting along the sewer well, and the maximum relative error between the simulated data and the measured data is only 2.5%. Taking the sewage pipe network of a town in Pearl River delta region for example, and further combining with mixed-node model, the variations of sewage COD concentration in municipal sewage system was also stimulated. The results show that all the parameters of the model are reasonably calculated and set and the model predicts the influent COD concentration of the sewage treatment plant quite well. The relative error between the simulated value and the measured value is 6.6%. Therefore, the model is able to make predictions and judgments on sewage COD concentration of the nodes, and is of benefit to the management of municipal sewage pipe network.