Thermochemical-mechanical damage prediction suitable for high-temperature and supersonic conditions is essential to evaluate the life span of barrel weapons. This paper proposes a thermochemical-mechanical damage prediction method in extreme environments by combining the cross-scale damage framework and scale expansion strategy. For the cross-scale damage framework, the macroscale surface damage is converted into mesoscale particulate impacts by the two-phase flow interior ballistics. The particulate impact is transformed into microscale crystal impacts by velocity decomposition and synthesis. For the scale expansion strategy, dislocation features of discretized crystals are obtained using the momentum mirror. The first proposed boundary dislocation can solve the boundary coupling of discretized crystals and modify the hardening criterion. A damage agent model is constructed based on sufficient samples to generalize mesoscale crystal damage to macroscale surface damage. The simulation experiment is executed to verify the accuracy of the calculation method for crystal impact damage under high-temperature supersonic environments. The launching experiment with 100 projectiles is executed to prove the accuracy of the thermochemical-mechanical damage prediction method.