The constant composition expansion experiments on natural gas with CO₂ content of 10%, 30%, 50%, 70%, 90%, 100% under three different temperatures （20 ℃, 30 ℃, 40 ℃） are conducted respectively. The results showthat with the CO₂ content increasing from10% to 100%, the critical temperature of natural gas samples increases from - 69.86℃ to 31.4 ℃, and the critical pressure increases from 4.85 MPa to 7.38 MPa. When the temperature and pressure is constant, with the increasing of CO₂ content in natural gas, the relative volume of natural gas samples shows an increase, and with the decreasing of pressure, the volume expansion lawof natural gas samples with different CO₂ content shows a similar phenomenon in the supercritical CO₂ characteristics. This cognition is significant for the further study on oil displacement by mixture of CO₂ and natural gas.