An air source heat pump water heater is a heat pump water heater that uses air as a low-temperature heat source to produce domestic hot water. It is mainly composed of an air source heat pump circulation system and a water storage tank. An air source heat pump water heater is a device that transfers heat from the air to water to produce hot water by consuming part of the electrical energy. In recent years, CO2 heat pump water heaters are being studied at home and abroad. [1] Literature [3] pointed out that under the condition of evaporation temperature 0℃, when water is heated from 9℃ to 60℃, the COP value of the CO2 heat pump hot water system can reach 4.3. When the surrounding air is used as the heat source, the average heating COP value of the whole year's operation can reach 4.0. Compared with traditional electric heating or coal-fired boilers, it can save 75% of energy.