Abstract: With the increasing demand for energy, high-temperature rock engineering such as the mining of deep underground solid mineral resources, geothermal energy extraction, deep nuclear waste repositories, and CO2 geological storage and solidification have been further developed. Therefore, it is necessary to accurately analyze the influence of high temperature and high pressure on the changing mechanisms of physical and mechanical properties of reservoir rocks. Based on the conventional triaxial compression tests of granite after high temperature ranging from 20 to 600℃ under different confining pressures, complete stress-stain curves of granite heated to various temperatures under conventional triaxial compression are analyzed and the influence of temperature and pressure on the deformation and strength characteristic and failure mode is discussed. Meanwhile, the mechanical changing mechanism of granite after exposure to various temperatures is revealed by optical microscopy observations. The test results show that: (1) Temperature has a significant effect on the expansion of volumetric strain, and the higher the temperature, the more obvious the volume expansion of the specimens. The triaxial compressive strength and elastic modulus of granite after high temperature gradually decrease with temperature, and increase with confining pressure. The cohesion and internal friction angle of granite after high temperature both decrease with temperature; (2) When the temperature is higher than 400℃, the changes in strength and deformation parameters of granite greatly increases, and the density and average width of microcracks also present a sudden increase trend overall. Meanwhile, the failure mode of the specimens under uniaxial compression condition changes from axial splitting failure to shear failure. The threshold temperature for the strength and deformation parameters and failure mode of granite is higher than 400℃; (3) Based on polarization microscopy observations, it was found that the escape of water molecules inside rock bodies, differences in mineral crystal expansion coefficients and mineral chemical changes lead to the initiation, propagation and interaction of microcracks between and within crystals in the granite, which ultimately causes the changes in the mechanical properties of the granite after exposure to various temperatures. The experimental results in this study is hoped to provide a theoretical basis for the design calculations and numerical simulations of high-temperature rock engineering projects.