Thermochemical–mechanical damage prediction suitable for high-temperature and supersonic conditions is essential for evaluating 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, macroscale surface damage is converted into mesoscale particulate impacts via two-phase flow interior ballistics. The particulate impact is transformed into microscale crystal impacts via velocity decomposition and synthesis. For the scale expansion strategy, the dislocation features of discretized crystals are obtained via 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 on the basis of sufficient samples to generalize mesoscale crystal damage to macroscale surface damage. A simulation experiment is executed to verify the accuracy of the calculation method for determining crystal impact damage under high-temperature supersonic environments. A launching experiment with 100 projectiles is executed to prove the accuracy of the thermochemical–mechanical damage prediction method.