Abstract: With the background of research on nonequilibrium flow field outside Mars probe, the thermal nonequilibrium process after temperature increase of a stationary CO₂ system from Martian atmospheric condition at an altitude of 30 km (181 K) is studied with state-to-state model. The system with initial condition of Martian atmosphere is heated suddenly to a high temperature and then is kept at constant temperature and volume. Three vibrational modes (symmetric stretching, bending and asymmetric stretching modes) and totally 201 vibrational energy levels are considered. The microscopic processes include: vibration-translation (VT) energy exchange processes that cause transitions between energy levels of the same or different modes, vibration-vibration (VV) energy exchange processes that cause transitions within the same mode. The time evolution of vibrational distribution for the three modes and the specifics of the corresponding microscopic processes are analyzed. The results show that: (1) VT processes are dominant, the contribution of VV processes can be ignored for such sudden increase and then constant temperature case. (2) Among the three vibrational modes, the bending mode has the fastest excitation rate and the largest equilibrium population. For the case of temperature rise to 2000 K, the relaxation time for the symmetric and asymmetric stretching modes is respectively about 2.2 times and 46.1 times of that for the bending mode. (3) As the transition rates increase with temperature, the average relaxation time for 5000 K case is lower than that for 2000 K by two orders of magnitude.