Abstract: Against the backdrop of the global energy transition and the “dual carbon” goals, underground coal gasification (UCG) is widely regarded as a strategic pathway for the in-situ, efficient conversion and clean utilization of deep, hard-to-mine coal resources. During the UCG process, the coal structure continuously evolves under high temperatures and multi-stage chemical reactions, such as drying, pyrolysis, gasification, and oxidation. These structural changes, particularly in pore morphology, exert a significant influence on gasifying agent flow behavior and syngas production rates. To elucidate the response mechanism of coal pore structures under thermal conditions, the effects of different atmospheres (nitrogen and air) and heating temperatures (200 ℃, 300 ℃, and 400 ℃) on porosity, pore size distribution, and pore connectivity were systematically investigated using low-field nuclear magnetic resonance (NMR) technology. The results indicate that thermal treatment promotes pore expansion and improved connectivity, as evidenced by the overall enhancement of transverse relaxation time spectra, with medium and large pores exhibiting the most pronounced increases. At the same temperature, the effect of thermal treatment in an air atmosphere is more significant than that in a nitrogen atmosphere. Thermal treatment under air notably increases the coal sample’s porosity, with the growth rate surpassing that under nitrogen; at 400 ℃, porosity increased by 90.04% in air compared to 65.24% in nitrogen. Moreover, as the temperature rises, the pore structure undergoes marked transformation: the proportion of micropores decreases, while that of mesopores and macropores increases. Thermal effects also enhance pore connectivity, thereby improving the permeability of the coal matrix. At 400 ℃ in air, the effective porosity reaches 19.17%, accounting for 75.56% of the total porosity, which is higher than the 15.56% and 70.53% observed under nitrogen, respectively. Furthermore, the fractal dimensions of medium and large pores show a decreasing trend with increasing temperature, indicating that thermal treatment renders the pore structure more regular, with simultaneous improvements in connectivity and permeability. During thermal treatment in an air atmosphere, the coal samples experience multiple stages including drying, gasification, and combustion, resulting in more severe internal structural damage.