The invention discloses a constant-temperature biogas tank with a sewage source heat pump, which belongs to the field of energy conservation and environmental protection. The constant-temperature biogas tank with the sewage source heat pump comprises a sewage source 1, a fully-automatic filter 2, a sewage circulating pump 3, a sewage heat exchanger 4, circulating water pumps 5, a sewage source heat pump unit 6 (an evaporator, a compressor, a condenser, and an expansion valve), a circulating water pump 7, a temperature sensor 8, a heating coil 9 and a biogas tank 10. The sewage source heat pump unit is connected with water return pipelines of sewage and water supply and return pipelines of hot water, and can realize the programmable control of a PLC; the sewage source is connected with the water supply and return pipelines of the sewage to form closed circulation; the circulating water pumps are arranged on the water return pipes of the sewage and the hot water respectively, and the automatic starting/stopping of the circulating water pumps is controlled by the sewage source heat pump unit; the heating coil is fixed on the inner wall of the biogas tank, and forms a loop with the sewage source heat pump unit through a heating pipe; and the temperature sensor is arranged in the dry fermentation biogas tank, monitors the temperature in the tank at any time, is in interlocking control with the heat pump unit, and controls the starting/stopping of the sewage source heat pump unit. The heating coil is fixed on the inner wall of the biogas tank. The invention can ensure that the biogas tank can operate all year around and is not affected by environmental factors and geographic factors, and can improve the biogas yield of the biogas tank.