Introduction to the Results: Transcritical CO2 heat pump systems can be widely applied in many fields such as domestic hot water supply, heating, and industrial drying. They also have significant advantages in environmental protection and good energy saving effects. With the tilt of national policies and the continuous deepening of research on CO2 heat pump technology in China, there will be a very promising development prospect for CO2 heat pump water heaters in our country. Currently, Xi'an Jiaotong University has cooperative projects with companies like Wanbao, Midea, Gree, etc., regarding heat pump water heaters and their compressors. Development Background: As people pay more attention to environmental issues, research on natural working fluids as refrigerants has gradually become a new research hotspot in the refrigeration industry. Due to the good thermodynamic properties of CO2, technologies using CO2 as a refrigerant for cooling and heating are also maturing step by step. Research on CO2 compressors is mainly conducted by some internationally renowned compressor manufacturing companies, such as Japan's SANYO, MYCOM, DAINKIN, and Italy's DORIN. In terms of research on transcritical CO2 heat pump water heaters, several Japanese companies have taken the lead in the world and introduced small household CO2 heat pump water heaters into the market in 2001. However, research and development on commercial CO2 heat pump water heaters have not yet resulted in mature models domestically or internationally. Advantages of the Results: Xi'an Jiaotong University initially focused on the research and development of commercial CO2 heat pump water heater compressors. In 2006, they developed a prototype of a commercial CO2 piston compressor. Through further collaboration with Guangzhou Wanbao Company, this model was further improved. The main parameters of the experimental prototype are: motor speed: 2900rpm (50Hz), rated input power of the motor: 18.0kW, theoretical volumetric flow rate of the compressor: 10.7 m³/h, maximum operating exhaust pressure: 13MPa, and maximum compression ratio: 8.5. Under the support of the National 863 Program, Xi'an Jiaotong University completed the design and installation of the prototype in 2008, and has now completed performance tests under various working conditions. The experimental prototype is an air source commercial transcritical CO2 heat pump water heater, which operates in an environmental condition of -20℃ to 40℃, with a maximum outlet temperature reaching 90℃ and a rated heating capacity of 55kW. At the same time, when the ambient temperature is -25℃, the prototype's outlet temperature can also reach 90℃. Its exhaust temperature can be controlled within 120°C.