<p>Deep rock is subjected to high temperature and high pressure, leading to shear failure behavior, which is often related to the safety and stability of engineering construction. Particularly in the real-time high-temperature true triaxial stress state (<i>σ</i>₁ &gt; <i>σ</i>₂ &gt; <i>σ</i>₃), the mechanism of rock shear failure requires further exploration. The true triaxial shear tests of intact grey sandstone under different normal stresses and real-time high-temperatures (25–280&#xa0;°C) were conducted. The characteristics of shear stress-deformation curves were analyzed in detail, and the evolution laws of strength, deformation, and mechanical parameters were obtained. Finally, through the macroscopic failure modes, combined with three-dimensional scanning technology and scanning electron microscope technology, the characteristics and laws of the microscopic morphology of the shear failure surface were analyzed. The true triaxial high-temperature shear failure mechanism of intact grey sandstone was further revealed. The results show that the peak shear strength of intact grey sandstone decreases under low normal stress (5&#xa0;MPa) and increases under high normal stress (30&#xa0;MPa, 40&#xa0;MPa). The increase in temperature leads to a decrease in cohesion and an increase in the angle of internal friction angle. The maximum normal deformation is highly sensitive to normal stress, and its value decreases with an increase in normal stress. The increase in normal stress can change the shear failure characteristics of rock, which is characterized by brittle failure under a low normal stress. The increase in thermally induced porosity is the main reason for the decrease in peak shear strength. Under high normal stress, it exhibits ductile failure characteristics, with noticeable friction scratches and transgranular failure. These findings provide a crucial foundation for comprehending and predicting the shear failure behavior of rocks in high-temperature environments.</p>