Deep/ultra-deep shale gas is a crucial target for future exploration and development. Currently, the maximum drilling vertical depth of some shale gas wells in southern Sichuan reaches 6600 meters, and the formation temperature reaches 180 ℃. Triaxial compression experiments with a real-time maximum temperature of 180 ℃ are carried out using actual shale cores from the Longmaxi-Wufeng formation with a depth from 5942.34 to 5951.75 meters below the surface, which is found in a region of southern Sichuan. A brittleness evaluation method based on the weight of mineral components and energy evolution is employed to reveal the mechanical properties and brittle characteristics of ultra-deep and high-temperature shale. The formation of brittleness is assessed based on the process. This approach allows for a more precise quantitative assessment of rock brittleness because it considers both the composition of the rock’s minerals and the development of energy during the whole failure process in a high-temperature environment. It is shown that the compressive strength of shales within the experimental setting temperature and confining pressure increases with temperature and/or confining pressure. The elastic modulus and Poisson’s ratio are less affected by temperature and confining pressure. With the increase of confining pressure, the failure mode of shale changes from brittle tensile-shear multi-crack failure to brittle-plastic single-crack shear failure. The calculated brittleness index of the ultra-deep shale interval is high, indicating that it has the physical basis for forming complex fracture networks.