<p>Thermal-induced changes in the mechanical behavior of organic soils are a critical concern in environmental engineering and geohazard studies, particularly under scenarios such as wildfires and geothermal development. However, their fracture and damage characteristics remain poorly understood. This study aims to elucidate the mechanisms by which high-temperature treatment affects the fracture toughness of soils with varying organic matter content. We subjected soil samples containing 1%–5% organic matter to heat treatments ranging from 100℃ to 600℃ and evaluated their fracture toughness using a triaxial creep apparatus. Results show that between 100℃ and 400℃, the release of structural water leads to an average reduction of approximately 48.8% in fracture toughness. In contrast, at 400℃ to 600℃, mineral expansion and partial melting help seal cracks, resulting in increased fracture toughness. Fracture toughness decreases with increasing organic content: samples with less than 3% organic matter show a reduction of about 16%, while those with more than 3% exhibit a greater reduction of up to 23%. Furthermore, high-temperature-induced oxidation and decomposition of organic matter generate numerous pores, facilitating crack propagation along pore networks. This study provides new insights into the damage evolution of organic soils under high-temperature conditions, offering theoretical support for post-fire soil stability assessment and ecological restoration.</p>