Abstract: With the further development of high-temperature rock engineering, it is necessary to accurately reveal the influence of high temperature and high pressure on the changing mechanisms of physical and mechanical properties of reservoir rocks to analyze the stability of high-temperature rock engineering reservoirs and improve resource extraction efficiency. 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 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, their changing mechanisms of mechanical properties of granite after exposure to various temperatures are revealed through optical microscopy observations. The test results show that: (1) The triaxial compressive strength and elastic modulus of granite after high temperature gradually decrease with temperature, and increase with confining pressure. Compared with room temperature, the uniaxial compressive strength and elastic modulus at 600℃ decreased by 56.47% and 54.63%, respectively, while under 60 MPa conditions, the triaxial compressive strength and elastic modulus only decreased by 41.04% and 33.51%, respectively. 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 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 conditions 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) The microcrack area (Φ_s) and average microcrack width (W_a) both gradually increase with temperature. At 400℃, Φ_s and W_a increase to 1.52% and 7.30 μm. As the temperature further rises to 600℃, Φ_s and W_a reach 3.67% and 11.00 μm, respectively, corresponding to the decreasing trend of mechanical and deformation parameters with temperature. It is 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 among and within crystals in the granite, which ultimately induces the changes in the mechanical properties of the granite. The combination of macroscopic and microscopic experimental results reveals the physical and mechanical changes in reservoir granite under high-temperature and high-pressure conditions, which provides provide a theoretical basis for the design calculations and numerical simulations of high-temperature rock engineering projects.