Abstract: With the increasing demand for energy and intensive attention paid on energy conservation and environmental protection，more and more studies have been made on CO2 heat pumps applied in refrigeration and heat pump. Compared with other heat-supply methods，heat pump is more efficient in heat supply.It can make use of a large amount of low-grade heat in environment with only a little high-grade heat consumption，which is called waste heat utilization，an ideal heat-supply system.As a natural working medium，CO2 is of prominent advantages for its good thermodynamic performance and conforming to clean and environmental requirements.Based on the introduction and advantage analysis on CO2 transcritical cycle， a thermodynamic cycle of a CO2 transcritical cycle was built and state points of the cycle were selected and studied in thermodynamic process.According to the national standard，CO2 transcritical cycle state points with various parameter were calculated，and the Coefficient of Performance of the heat pump （ COPh ）was increased to 3.793 by optimization. The results show that transcritical cycle not only improves the heating efficiency of the system，but also compresses the equipment space，which has great development and improvement potential.

Key words: CO2 heat pump, CO2 transcritical cycle, heat pump heat supply, thermodynamic cycle, energy saving and environmental protection, energy