<p>This paper experimentally examines the impact of high temperatures on the physical and mechanical properties of pre-damaged sandstone. Nuclear magnetic resonance (NMR) technology is utilized to observe pore evolution under combined preloading and thermal treatment. Uniaxial compression tests were conducted to evaluate the strength variation and damage mechanisms, while fractal dimension theory was applied to characterize pore structure based on NMR data. The results showed that high temperature and preloading treatments have a great influence on the internal pore characteristics of sandstone samples. After 400&#xa0;°C treatments, the proportion of mesopores and macropores of sandstone decreases, while the proportion of micropores increases. After 600&#xa0;°C treatments, the proportion of micropores and macropores of sandstone decreases, and the proportion of mesopores increases. The reduction of compressive strength of sandstone after 400 ℃ is mainly controlled by mechanical damage, while the reduction of compressive strength of sandstone after 600℃ is mainly controlled by thermal damage. The thermal damage of sandstone is related to the increase in the proportion of mesopores, which leads to the low strength of sandstone after 600℃ and decreases significantly with preloading ratio (The compressive strength decreased from 107.4&#xa0;MPa to 72.8&#xa0;MPa as the preloading ratio increased from 0.4 to 0.8). The larger the preloading ratio of sandstone after high temperature treatment, the smaller the fractal index <i>D</i>_NMR. And the <i>D</i>_NMR of sandstone after 600℃ is roughly positively linearly correlated with porosity, and the <i>D</i>_NMR of sandstone after 400℃ is roughly negatively linearly correlated with porosity.</p>