Abstract: During the exploitation of deep geothermal resources, the thermal fractures of high-temperature rocks are usually induced by the impact of low-temperature fluids to improve the permeability of reservoir rocks. In order to reveal the damage and fracture mechanism of rock after thermal shock, the granites heated at high temperature (20 ℃, 150 ℃, 300 ℃, 450 ℃, 600 ℃ and 750 ℃) were treated by natural cooling and water cooling respectively, and the wave velocity test, uniaxial compression test and CT scanning were carried out on the treated granites. The mechanical effect of thermal shock on P-wave velocity, compressive strength and elastic modulus of granite were also discussed. The experimental results show that with the increase of heat treatment temperature, the P-wave velocity, compressive strength, and elastic modulus of rock gradually decrease, and the peak strain gradually increases. Compared with natural cooling, the wave velocity and mechanical properties of rock deteriorate more significantly after water cooling. Based on CT scanning, the spatial distribution characteristics of pore and fracture structure of granite under different heating temperatures and heat treatment methods were obtained, which can directly reflect the thermal damage degree of rock microstructure. When the heat treatment temperature is not higher than 450 ℃, the number and size of thermally induced cracks in granite scanning slices are less and the connectivity of cracks is relatively poor. When the temperature exceeds 450 ℃, the micro-cracks in granite develop and expand rapidly, and tend to form fracture network gradually, and the damage and cracking effect of water cooling on the microscomic-structure of granite is more obvious than that of natural cooling. In addition, based on triangular mesh discretization technique, ellipsoid model reconstruction algorithm and fracture tensor calculation theory, the three-dimensional fracture field of granite after thermal shock is quantitatively characterized, and the relationship between fracture fabric tensor and peak strength was established, which further reveals the influence mechanism of granite microscomic-structure on its mechanical properties under thermal shock.