Abstract: Understanding the evolution of the microstructure and mechanical properties of granite under high temperature conditions is of great significance for the stability evaluation of rock engineering related to safe disposal of nuclear waste, geothermal mining, and tunnel fire repair. To quantitatively study the effect of temperature on the pore structure of granite, CT scanning and polarizing microscopy were used to analyze the microstructural characteristics such as fracture evolution, porosity, and pore size distribution of thermally damaged granite. The research results indicate that as the temperature increases, the uniaxial compressive strength and elastic modulus of granite gradually decrease, and the uniaxial compressive strength of granite significantly decreases after 500 ℃; High temperature can promote the development of granite pores and cracks, and enhance the connectivity of pores; The deterioration of mechanical properties of granite after high temperature is closely related to the changes in its pore structure, and the mechanical strength of granite decreases with increasing porosity; Granite undergoes physical and chemical processes such as water evaporation, mineral oxidation, and chemical bond fracture at high temperatures, which can cause defects such as grain boundary cracks, intragranular cracks, and transgranular cracks in mineral particles. The damage to rock structure caused by quartz phase transformation is particularly significant.