To investigate the formation of fouling in sewage heat exchangers, a model of thermal resistance with time is developed based on the experiments in a practical engineering site. And this model is asymptotic exponential function. According to the characteristic of sewage heat exchangers, the effective thermal resistance and effective coefficient of heat transmission during the formation of fouling are defined. A model for pressure loss of fouling of asymptotic distribution is presented based on the model of thermal resistance of fouling. And the maximum absolute margin of error of the fouling thermal resistance is smaller than the typical allowable error range. The maximum relative error of the heat transfer coefficient is 12%. These can meet the requirements of engineering. The results of experiments provide a basis for further study and application of swage heat exchangers.