Based on the EOS-CG model and GERG-2008 model, the density of the mixed carbon dioxide gas containing impurities is calculated, and the viscosity of the mixed gas is calculated based on the CO2-Pedersen model. The relationship between density, viscosity and pressure of pure carbon dioxide and mixed gas are obtained by fitting the calculated data of these models to describe the real gas behavior and the law of viscosity changing with pressure of pure carbon dioxide and mixed gas. The steady-state Reynolds equation is solved by using the finite difference method, and the opening force, leakage rate and film stiffness of the dry gas seal lubricated by pure carbon dioxide or carbon dioxide with impurities are obtained, and the influence of impurities on the performance (opening force, leakage rate, film stiffness) is also analyzed. The variables considered include the average linear velocity of the end face, film thickness, inlet temperature, and inlet pressure. The results show that when inlet pressure is 15.26 MPa, and inlet temperature is 363.15 K, and linear velocity of the end face is 74.030 m/s, and film thickness is 3.05 μm, both the opening force and leakage rate of the dry gas seal lubricated by carbon dioxide with impurities are lower than those of pure carbon dioxide dry gas seal. The more impurities content, the more obvious the difference. The effect of impurities on the opening force, leakage rate, and film stiffness of the carbon dioxide dry gas seal increases with the increasing of the averaged linear velocity of the end face. The effect on the leakage rate and film stiffness decreases with the increasing of the film thickness. The effect on the opening force, leakage rate, and film stiffness decreases with the increasing inlet temperature. The influence on the opening force of carbon dioxide dry gas seal decreases firstly, then increases, and finally decreases with the increasing of the inlet pressure, and the influence on the leakage rate of carbon dioxide dry gas seal increases firstly and then decreases with the increasing of the inlet pressure, and the influence on the film stiffness decreases firstly and then increases with the increasing of the inlet pressure.