Igneous intrusions are distributed extensively in the Huaibei coalfield, China. In the Haizi coal mine, coal and gas outbursts have occurred 11 times under an extremely thick sill (average thickness 120 m). This paper presents the results of a study on the influences of the igneous rock on coal pore structure, methane desorption and diffusion properties, and coal seam gas occurrence. The results show that the thermal evolution effect of the igneous sill prominently increases the specific surface area and pore volume of the affected coal. Samples HZ1 and HZ2 (No. 7 and No. 9 seams, respectively) closer to the sill possess improved pore connectivity, while samples HZ3 and HZ4 (away from the sill) and sample HZ5 (without sill covering) of the No. 10 coal seam have poor pore connectivity. Moreover, the effective diffusivity and desorption indexes of the coal increase progressively closer to the sill. The thermal effect of the igneous sill promotes the development of coal pores, thus leading to better pore connectivity, more desorbed gas, and much higher gas desorption and diffusion rates. Consequently, the thermal evolution effect of the igneous sill can change the occurrence and characteristics of the entrapment effect in the underlying coal seams, thus resulting in a high probability of gas hazards or even coal and gas outbursts in the coal seam close to the igneous sill. Engineering practices show that the affected coal seams have high gas content, gas pressure, and gas emission amounts as well as a high propensity for coal and gas outburst.