Abstract: Existing studies on the spontaneous combustion of coal gangue mountains mainly focus on constructing simplified airflow seepage models, often neglecting the effects of internal convective heat transfer, stacking forms, and spatial distribution characteristics of porosity on the temperature field of spontaneous combustion in coal gangue mountains. To address these issues, a multi-field coupled numerical simulation of the temperature, seepage, and oxygen concentration fields during the spontaneous combustion process of coal gangue mountains with two different stacking forms—dumped along the slope and piled up on flat ground—was conducted using Fluent software. The spatiotemporal evolution patterns of oxygen concentration and temperature fields were analyzed. The results showed that: ① the internal oxygen concentration of the coal gangue mountain decreased rapidly at first and then gradually with increasing distance from the slope surface and top surface. The area of low-oxygen-concentration regions increased gradually with time, while the average internal oxygen concentration decreased over time with a gradually decreasing rate of decline. ② The internal temperature of the coal gangue mountain increased over time, and the heating rate gradually decreased. In both horizontal and vertical directions, the temperature exhibited similar variation patterns—it rose rapidly at first and then slowly decreased with increasing distance from the slope surface and the bottom surface. The high-temperature region was mainly concentrated at a distance of 1–6 m from the slope surface and at 40%–90% of the vertical height (measured from the bottom of the coal gangue mountain). ③ The surface temperature of most regions of the coal gangue mountain was below 100 ℃, with only a few areas ranging from 100 to 110 ℃. The temperature variation trends of the slope surface and the top surface were consistent, but the top surface temperature was slightly higher than that of the slope surface.