<p>The thermal shock effect, a consequence of temperature fluctuations in geothermal development, significantly influences the mechanical properties of hot dry rock (HDR). This study experimentally investigates the degradation of these properties following rapid temperature drops and introduces a calculation method for assessing the stability of HDR reservoirs. Through triaxial compression tests, it is demonstrated that both stress and temperature substantially affect the mechanical behavior of HDR, with stress shown to inhibit thermal crack propagation and temperature changes accelerating strength degradation. A nonlinear coefficient λ, derived from the Mohr–Coulomb failure criterion, quantifies the rock's degree of thermal damage. A mathematical relationship between <i>λ</i>, uniaxial compressive strength (<i>σ</i>_c), and the critical confining pressure coefficient (<i>η</i>) is established, providing a novel perspective on the nonlinear failure criterion under thermal shock conditions. The experiments validate the accuracy of the proposed criterion, offering a robust framework for predicting the behavior of HDR under various thermal conditions. This research advances the field of geothermal reservoir engineering by providing a precise tool for assessing rock stability in response to thermal shock.</p>