Urban rainfall-runoff pollution is one of the important reasons for the deterioration of urban water quality. Quantitative calculation of urban rainfall-runoff pollution load is the basis and key for implementing total urban water environmental pollution control management, which can provide scientific basis for urban water environment management and pollution control. Based on the theory of pollution build-up and wash-off, this paper put forward the concept and calculation formula of “the characteristic area”, and established a mathematical model of pollution load from each rainfall runoff. Combined with the case studies, the validity, accuracy, applicability, and limitation of the mathematical model were evaluated. The results showed that the characteristic area better reflected the pollution load characteristics on various types of land, the rainfall-runoff pollution load was proportional to the product of the characteristic area and rainfall. The model established in this paper could be calibrated by 3 or more sample data of rainfalls, so that the rainfall-runoff pollution load could be estimated quickly and more accurately. This method was simple and practical and could be used in a wide range of areas, furthermore, it could greatly reduce the workload of data acquisition. For small rainfall events, it was recommended to calibrate the model with observation results of similar rainfall to improve the prediction accuracy of the model.