The study of deformation behavior and damage characteristics under coupled thermal-mechanical loading is crucial to comprehensively understanding of the progressive failure law of rock mass in the high-level radioactive waste repository. The aim of this study is to investigate the macro-deformation behavior, damage evolution process, and failure characteristics of Beishan granite after thermal treatment. Granite samples are from Gansu Province a potential site for China's HLW disposal. Triaxial compression tests coupled with transmitted cross-polarized light and X-ray diffraction (XRD) micro-observation are performed on Beishan granite samples under different confining pressures. The mass, density, P-wave velocity and peak strength gradually decrease, while the volume and the peak axial strain slowly increase with the increase of temperature. Accordingly, the peak strengths decrease by 77.70%, 57.28%, 37.33% and 33.97% respectively, while peak strains increase by 196.37%, 115.27%, 105.13% and 90.38% at the 1,000 ℃ under different confining pressures. At the low temperatures and low confining pressures, the failure form of sample is characterized by tensile split, and a number of coalescent axial fissures around fracture surface are observed. The deformation pattern transforms from brittle fracture to plastic shear deformation with steep dip as the temperature and confining pressure increase.