<p>Sewage source heat pump technology can use the heat in sewage to heat and cool buildings. At present, the main problem of this technology is scaling. Microbial fouling and particulate fouling are the main components of sewage heat exchanger fouling. In this paper, the mixed fouling composed of microbial fouling and particulate fouling was studied. The deposition characteristics of mixed fouling in sewage were studied by numerical simulation. The mathematical model of mixed fouling deposition was established. The model was used to simulate and analyze the influence of various factors on mixed fouling deposition. The results show that when the flow rate increases, the percentage reduction of thermal resistance of mixed fouling equilibrium in winter is slightly lower than that in summer. The higher the inlet temperature, the higher the deposition rate of mixed fouling, and the shorter the time required for the thermal resistance to reach equilibrium. When the particle size of microbial particles and the size of hard particles increase, the percentage increase of thermal resistance when the mixed fouling reaches equilibrium in winter is slightly higher than that in summer. With the increase of microbial particle concentration, the increase of thermal resistance of mixed fouling in winter is lower than that in summer. With the increase of hard particle concentration, the increase of thermal resistance in winter is lower than that in summer. The research results of this paper can provide scientific basis for the design of sewage heat transfer system, and have important engineering application value.</p>