Abstract: Research into the thermal damage of rocks is crucial for assessing the consequences of tunnel fires, ensuring the safety of nuclear waste disposal, and advancing deep engineering fields such as underground coal gasification. This study systematically analyzes the effects of two heating methods—thermal conduction heating and microwave heating—on the color, impedance, and mode I fracture toughness of granite centrally cracked straight through Brazilian disc（CSTBD） specimens at various temperatures. The aim is to reveal the differences in rock thermal damage mechanisms under these two heating conditions. The results indicate that regardless of the heating method, as the temperature increases, the granite’s color and impedance gradually rise, while its fracture toughness progressively decreases. An analysis of damage variables, defined by cumulative acoustic emission ringing counts, shows that both thermal conduction and microwave heating promote the development of microcracks within the granite, thereby intensifying thermal damage. A notable distinction lies in the crack development patterns. With thermal conduction heating, intergranular and transgranular cracks only begin to develop on the rock surface when the temperature reaches 500 ℃. In contrast, microwave heating leads to significant crack development at a surface temperature of merely 300 ℃. Furthermore, under microwave irradiation, the crack morphology transforms from straight to curved, exhibiting more severe deviation and tortuosity. Under microwave irradiation, water molecules within the granite undergo multiple phase transition cycles. Simultaneously, minerals with high dielectric constants （such as biotite） selectively and rapidly absorb microwaves, leading to significant differential thermal effects and localized high-temperature regions inside the rock. This effect causes the actual internal temperature around the microwave-absorbing minerals to be much higher than the measured surface temperature （especially after the surface temperature reaches 150 ℃, where fracture toughness significantly decreases）. The resulting immense internal temperature gradients and thermal stress concentrations drastically increase the density of microcracks within mineral grains and exacerbate overall thermal damage, leading to earlier crack initiation and more tortuous propagation paths. The findings of this study offer significant guidance for comprehensively understanding and predicting the mechanical response and damage mechanisms of rocks in complex high-temperature environments.