Abstract: Mars probe usually is an obtuse vehicle with a spherical-crown/inverted-cone configuration. For this kind of vehicles, the aft-body flow is complex, such as large scale separation and reattachment, shock/vortex interaction. Consequently, the dynamic characteristics are also extremely complex and become one of the key problems during the design process. In this paper, the approximate specific heat ratio method is used to simulate the atmospheric environment of Mars for ballistic range Mars probe test model(MSL model). Based on the verification of the numerical method with in-house software FLY3D-Mars, the variation of pitching dynamic characteristics with specific heat ratio are numerically studied. The results show that the experimental configuration is dynamically stable in all the investigated ranges. As the specific heat ratio decreases, the compressibility becomes stronger, the dynamic derivative reduces and the dynamic stability can be improved accordingly. The reason for the change of dynamic stability with specific heat ratio is that the compressibility of the flow is different under the conditions with different specific heat ratios, which is consistent with that of ballistic range test.