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When the loss of coolant accident happens in a reactor, because of the high temperature and high pressure in the reactor and the low external pressure, the critical flow may occur at the break due to sharp depressurization and vaporization. The critical flow characteristics have a great impact on the accident process, and the accurate estimation of critical flow at the break is more important for the safety analysis of the supercritical water reactor. In order to obtain the critical flow characteristics under supercritical conditions, the steady state tests of critical flow were systematically carried out under supercritical pressure in a nozzle test section with a diameter of 2 mm and a length diameter ratio of 1-20, taking supercritical CO2 as the working fluid. The test pressure range was 7.4-9.5 MPa and the temperature range was 15-55 ℃, a large number of reliable test data were obtained from the test. The effects of stagnation pressure, stagnation temperature and nozzle length diameter ratio on critical flow were obtained. That is, when the pressure is constant, the critical flow decreases with the increase of inlet temperature, and the critical flow drops sharply at the beginning, but when the inlet temperature is higher than a certain value, the critical flow changes little with the increase of stagnation temperature. The turning point of flow rate changing with temperature is about the pseudo critical temperature of the corresponding pressure. The higher the stagnation pressure, the greater the critical flow. The influence law of stagnation temperature and stagnation pressure on critical flow is basically consistent with the change law of critical flow of water. For short nozzle and medium long nozzle, the influence of length diameter ratio is small. For long nozzle, due to the greater influence of friction resistance, the flow is much smaller, but the higher the stagnation temperature, the higher the stagnation pressure, the smaller the influence of the length diameter ratio. The obtained experimental data complements the critical flow experimental databases and provides experimental data for the development and verification of the critical flow model. With the obtained supercritical CO2 test data, the universality and accuracy of the general thermal equilibrium discharge flow model are further verified, which is applicable to the critical flow of supercritical CO2 and can better predict the critical flow under supercritical conditions.