Abstract: In order to study the characteristics and mechanism of thermal damage of the granite, the analyses of attenuation law of longitudinal wave, damage evolution model and the mesoscopic mechanism of rock under different temperatures are carried out by using the ultrasonic testing analyzer and tungsten-filament scanning electron microscope (SEM). The analysis results show that when the temperature is lower than 500 ℃, the longitudinal wave velocity of the granite sample slowly decreases with the increase of treatment temperature; and when the temperature is near 573 ℃, the wave velocity drops rapidly and is gradually tends to be stable at last. With the increase in heated temperature, the amplitude of wave declines, the length of "beat" becomes shorter and the relationships between the thermal damage and the heated temperatures well follow the logistic curve model. In the range of 20 ℃ to 500 ℃, the inter-granular crack predominates; and as the temperature rises to 700 ℃, the number of microcracks increases, and the intracrystalline and transgranular crack occur. When the treatment temperature reaches up to 900 ℃, the macroscopic branching fissure appears in the sample. The research results can provide a referrence for construction of rockmass engineering after higher temperature.