Abstract: The chemical non-equilibrium has a strong impact on the aerodynamic characteristics of re-entry vehicles flying at high Mach number, which leads to a high temperature shock layer. The altitude and velocity variations would affect the chemical non-equilibrium, and in turn would affect the aerodynamic changes. The numerical investigation and the mechanism analysis are carried out for the re-entry vehicles by solving a three dimensional Navier-Stokes hydrodynamics equation, to understand the effect of the chemical non-equilibrium on the aerodynamic characteristics, as compared to the perfect gas model. The good agreement between the calculated results and the reference data for AS-202 flight test validates the model and the numerical methods. The results show that the predictions of the perfect gas model remain the same and the differences of the aerodynamic forces between the perfect gas model and the chemical reaction gas model are increased with the increase of the Mach number under unchanged altitude. That is because the increase of the Mach number strengthens the chemical non-equilibrium effect. With the increase of the altitude under the same Mach number, the differences between these two gas models show the same trend, the aerodynamic force and the trim angle deviations increase slightly. That indicates the increase of the altitude also strengthens the chemical non-equilibrium effect. It is found that the change of the flow structure under the chemical non-equilibrium and the flucturation of pressure distribution under different flight conditions is the main reason of the aerodynamic variations.