Objective: This study aimed to optimize the performance of heat pump system and discuss the optimal exhaust pressure of carbon dioxide system under the control of different working conditions. Methods: By establishing a CO₂ trans-critical heat pump model, the relationship between exhaust pressure and heat production, inspiratory superheat, refrigerant mass flow rate and system energy efficiency was analyzed, and the influence of inlet and outlet water temperature and ambient temperature on the optimal exhaust pressure was deeply studied. Moreover, the reliability of the model was verified by the CO₂ heat pump test bench. Results: At the three effluent temperatures, the heat production increased with the increase of exhaust pressure, and the mass flow rate decreased with the increase of exhaust pressure. The cycle performance coefficient and superheat showed a trend of first increasing and then decreasing with the increase of exhaust pressure, the optimal exhaust pressure increased with the increase of inlet and outlet water temperature and ambient temperature, but the increase of inlet and outlet water temperature reduced the system cycle performance coefficient. Based on this, the optimal exhaust pressure correlation formula at the outlet temperature of 60 ℃ was fitted, and five groups of tests were designed to prove its reliability. Conclusion: The optimal exhaust pressure of the system is determined by a large number of simulation data, and compared with the test values, it is found that the error values are less than 5%, which meets the test and design requirements.