The work aims to conduct a theoretical study on the closed drying system powered by transcritical CO2 heat pump and obtain the method and principle of calculating the optimal working conditions of CO2 closed heat pump. A mathematical model of thermodynamic couple between CO2 cycle and air cycle was established. Based on the model, the temperature, enthalpy, relative humidity and moisture content of air in the drying cycle were calculated and the temperature, pressure and enthalpy of working medium in transcritical CO2 heat pump system were figured out. The coupling mechanism between air cycle and CO2 heat pump cycle was explored by varying the air temperature after condensation and drying and taking COP in the heat pump drying system as the evaluation basis. The optimal CO2 discharge pressure in the closed drying system under different working conditions was obtained. Based on the established calculation program, the thermodynamic parameters of heat pump system under typical process parameters were obtained, providing a theoretical basis for the selection of key equipment (fans, heat exchangers, compressors, etc.) and system control methods. The research shows that the COP of the system is optimal when the outlet of CO2 heat pump cooler is at critical state.