Bathing was tewater contains a large amount of neaste heat, a double heat pump unit in series with large temperature difference was proposed to maximize the recovery of waste heat energy from bathing wastewater. The rated design conditions of the unit are as follows, the inlet and outlet temperatures of the bathing wastewater on the evaporator side are 30℃ and 6℃、respectively and the inlet temperature of tap water on the condenser side is 10℃ with hot water outlet temperature at 45℃. In this study, the effects of inlet temperature of tap water, outlet temperature of hot water and inlet temperature of bath wastewater on thermal performance of heat pump unit were studied experimentally. The experimental results showed that the COP reduced from 5.0 to 3.85 as the inlet temperature of tap water increased from 5℃ to 15℃. When the outlet temperature of the hot water changed from 40℃ to 50℃, the COP reduced by 7.5%. And when the inlet temperature of bathing wastewater rose from 30℃ to 35℃, the COP increased by 3.1%. In the absence of an auxiliary heat source, if the bathing wastewater temperature reduces from 30℃ to 6℃, the tap water temperature must be higher than 12℃ to ensure that the hot water outlet temperature is 45℃. Besides, when the inlet temperature of bathing wastewater reaches 35℃, the hot water outlet temperature can also reach 45℃ even if the tap water temperature is 6℃.