<p>This study investigates the fracture behavior of clay-rich mudstone under varying temperature and pressure conditions, which is crucial for the safety of geological structures. It focuses on three fracture types: pure mode I tensile fractures, pure mode II tensile fractures, and shear fractures, examining specimens at room temperature (RT) and after thermal treatments at 250 and 500 °C. The findings reveal that increasing temperatures makes the mudstone more brittle, enhancing fracture velocity, toughness, load-bearing capacity, roughness, and the fracture process zone (FPZ) radius. Notably, tensile fractures induced under pure mode II displayed the highest velocities, while shear fractures exhibited the lowest velocities, smoothest surfaces, and greatest resistance to failure. The application of a confining pressure of 4 MPa significantly improved shear fracture toughness by 119.7%, 98.5% and 71.9% at RT, 250 °C and 500 °C, respectively, and reduced roughness by 8.2%, 22.4% and 30.4%. This research offers a novel, comprehensive view of how temperature and pressure impact fractures in mudstone sensitive to temperature due to its high clay content and water affinity. The findings provide valuable insights applicable to geothermal energy, oil and gas exploration, and underground construction, thereby enhancing the understanding of fracture mechanics in geological contexts.</p>