A shell-tube sewage heat exchanger with auto-de-fouling function was first developed, followed by the modeling of its distributed model for the dry-type shell-tube sewage evaporator according to the theory of two-phase flow and thermodynamics of refrigeration systems. Then the model was solved by combining iterating two-side fluids with unidirectional solving of single-side flow. By comparing the simulation results with the experiment data, the accuracy of the simulation program was proved. The results have shown that the maximum refrigerant pressure drop in the evaporator is 0.34 kPa.