The supercritical flow of CO2 is widely applied in energy fields such as the carbon capture and storage technology and the Brayton cycle power generation technology. The shut-off valves are widely used in various systems of energy fields, and their use is related to the stability and safety of the system. Due to the throttling effect of supercritical CO2 flowing through the valve, the large temperature drop may cause the dry ice generation, the overpressure and the blockage, which have an important impact on the safety and stability of the whole system. At present, there are few researches on the throttling effect and thermodynamic characteristics of supercritical CO2 flowing through the shut-off valve. In this paper, the pressure and temperature changes before the valve inlet and after the valve outlet were studied, and the field of pressure, temperature and phase inside the valve were analyzed. It was found that the outlet pressure of the shut-off valve increased rapidly after the valve was opened, and the temperature decreased at the same time. With the decrease of system pressure, supercritical CO2 in front of the valve changed into gas-liquid two-phase and gas-phase successively, and CO2 after the valve changed into gas-phase, liquid-phase, gas-liquid two-phase and gas-phase successively. This study is of great significance to the stability and safe operation of energy systems.