Abstract: The thermal rupture and damage mechanism of rock has received more and more attention in the high-level waste geological disposal project. The granite rock samples from the high-level waste disposal repository in Beishan, Gansu Province,China were collected in this study. The thermal stability of granite was investigated by low-field nucle- ar magnetic resonance,MTS rock mechanics test machine and inverted optical microscope. It was found that ① NMR T2 spectrum has no significant change from 0 ℃ to 400 ℃ . When the temperature is higher than 500 ℃ ,the amplitude of the T2 spectrum increases significantly and shifts to the right. The T2 spectral area and porosity show a trend of power-law during heating;② As the increase of temperature and porosity,the peak-stress decreases with power-law rela- tionship;③ It was found by magnetic resonance imaging (MRI) that the proton density distribution is homogeneous and no obvious proton density clusters appear below 500 ℃ ,explaining the internal structure of granite is stable. When the temperature is higher than 500 ℃ ,a large number of high proton density regions caused by the occurrence of crys- tal and boundary cracks,and the micro-regions with high proton density merge into a large connected region as the temperature increases continuously;④ The probability density function of the MRI pixel of granite obeys the lognormal distribution under different temperatures,the probability density function moves to the right with the occurrence of α /β phase transition;⑤ According to microstructure observation,boundary cracks generate between feldspar crystals or between feldspar grains and quartz grains at 500 ℃ ,and a small amount of transgranular-cracks appear inside the crys- tals. The quartz particles produce large-area transparent transgranular cracks accompanied by a distinct transgranular network when the temperature exceeds the α / β phase transition point.