To investigate the influence of high temperature on the sealing performance of mudstone caprock during underground coal gasification (UCG), experimental studies were conducted on the gas seepage behavior of mudstone under temperature-stress coupling conditions. The mudstone caprock from the Taiyuan Formation-Shanxi Formation coal seam roof in the Ordos Basin was used as the research object. The effects of temperature (20, 200, 400, and 600 ℃) on the physical properties and seepage behavior of the mudstone caprock were analyzed, and a mathematical model was established to describe the relationship between temperature-stress coupling and mudstone gas permeability. The results indicate that a critical temperature of 400 ℃ causes a significant change in the permeability behavior of mudstone. Compared with the condition below 400 ℃, the mass loss rate, elongation, thermal expansion coefficient, and gas permeability of the mudstone increase more markedly with rising temperature, and its mesostructure is more severely damaged. Beyond this critical temperature, the gas-sealing performance of the mudstone caprock is greatly reduced, which adversely affects the gas collection efficiency of the underground coal gasification chamber. The findings reveal the significance of temperature-stress coupling effects on the gas permeability of mudstone, which can provide an important theoretical foundation and parametric support for evaluating the sealing capacity of caprocks, predicting gas leakage risks, and controlling the stability of surrounding rocks under high-temperature disturbances during underground coal gasification (UCG). This holds practical implications for advancing the engineering application of UCG in China.