The entire process of slowly raising granite from room temperature to 600℃, natural cooling in air, and thermal shock cooling in water followed by cooling to room temperature was observed using a polarizing microscope. The microscopic structures were analyzed by quantitatively analyzing the mineral area and the number of cracks in the field of view to investigate the damage caused to granite during the heating process and after thermal shock in different cooling media. The results indicate that:(1) The mineral area of granite increases with temperature during the heating process. After natural cooling from high temperature, the mineral area is slightly larger than before the heat treatment, while after thermal shock in water, the mineral area is slightly smaller than at 600℃but significantly larger than before the heat treatment.(2) During the heating process, the overall trend of cracks in the field of view shows an increase in small cracks, followed by the formation of large cracks through the penetration of small cracks, and a continuous increase in both large and small cracks. The critical temperature for a decrease in small cracks and an increase in large cracks is around 300℃.(3) After thermal shock in water, the total number of cracks in the high-temperature granite increases, with a significant increase in cracks between 0-100μm. In contrast, after natural cooling in air, the total number of cracks in the granite decreases, and there is a significant decrease in cracks between 0-100μm. This indicates that shorter and smaller cracks are more sensitive to the cooling process.(4) Through analysis of the degree of mineral contraction and changes in the number of cracks, it is found that internal fracturing caused by thermal shock cooling in water is more severe in high-temperature granite.