In this paper, an Isochoric-Burnett combination method has been proposed to measure the PVT properties of H₂-CO₂-H₂O mixture at high temperatures and high pressures up to 650 °C and 22 MPa. A fast isochoric measurement process has been used to handle the problem of hydrogen leakage at high temperatures and a series of expansion measurement processes have been used to obtain the mass data of the gas mixture. This method can avoid the leakage of hydrogen, obtain the mass data of the H₂-CO₂-H₂O mixture precisely, and further analysis shows that uncertainties of temperature, pressure and density data are not above 10 mK, 0.9% and 0.5%.