Abstract: In the process of underground coal gasification and geothermal energy mining, the change of high temperature environment and stress field will affect the mechanical properties and strength characteristics of rocks, and the study of the change of the strength and mechanical properties of the rocks after high-temperature treatment under the action of different triaxial stresses has a very important role in the practical application of underground engineering. Based on the dynamic monitoring test system of hydraulic wetting range of coal rock under true triaxial stress, the loading and unloading tests of sandstone under different temperatures and different triaxial stress conditions were carried out. The deformation characteristics, strength characteristics and energy variation law of sandstone after high temperature were analyzed, and the effects of temperature and triaxial stress on the macroscopic strength of sandstone were studied. The failure of sandstone is dominated by brittle failure. The failure surfaces of sandstone at different temperatures are formed along the direction perpendicular to the minimum principal stress. The fracture angle increases with the increase of temperature and tends to be vertical. The bearing capacity of sandstone increases first and then decreases with the increase of temperature. The bearing capacity reaches its maximum at 600 ℃ and its minimum at 1 000 ℃. The influence of temperature on the elastic modulus, deformation modulus and other deformation parameters of sandstone is obvious, and the influence of triaxial stress on the deformation parameters is relatively small. The deformation parameters of sandstone are negatively correlated with temperature, and the deformation parameters of sandstone are positively correlated with triaxial stress. At 1 000 ℃, the elastic modulus, deformation modulus and peak strength of sandstone are the smallest, and the ultimate strain is the largest. During the loading and unloading process of sandstone, the change of energy density is roughly the same as that of peak strength, and the higher proportion of elastic energy to input energy has a greater impact on sandstone failure. Combined with the changes of mineral composition, pore fissure structure and porosity of sandstone, it is found that the macroscopic strength change of sandstone is consistent with the microscopic structure change.