Under extremely high-temperature environments, thermal cracking damage on component surfaces becomes markedly pronounced, significantly increasing surface roughness and reducing component lifespan. To analysis this issue, a dynamic computational model for thermal crack initiation and propagation under thermo-mechanical coupling was established based on Fourier’s heat conduction theory and thermal stress theory, employing a finite-discrete element method (FDEM). Experimental validation confirmed the model’s accuracy. Results demonstrate that material high-temperature mechanical properties govern the thermal cracking process. At elevated temperatures, particularly when approaching the material’s melting point, tensile strength undergoes dramatic degradation, with surface tensile strength approaching zero. During subsequent cooling, thermal cracking becomes particularly pronounced. The thermal cracking evolution exhibits three distinct stages: Initial stress concentration leads to the formation of micro-cracks on the surface. Subsequently, these micro-cracks extend inward with gradually decreasing propagation rates. Eventually, crack propagation stops and crack opening becomes dominant, resulting in progressive widening of the cracks. This mechanism fragments the material surface into discrete blocks. The developed cracks effectively redistribute the thermal strain. In the subsequent process, the formation of new micro-cracks on separated fragments is suppressed, and the fragment shrinks, causing the material surface to be concave inward.