Abstract: To study the thermal effect of microwave on sandstone in coal roof at microscopic level, rock samples were pretreated before microwave experiment, and the microstructure before and after microwave radiation was investigated by nuclear magnetic resonance（ＮＭＲ） and scanning electron microscopy（ＳＥＭ）. The results show that microwave radiation is very conducive to increase the pore number, enlarge the pore volume and enhance the pore connectivity. With the propagation of microwave, the pore modification is obviously weakened. The intergranular fractures and transgranular fractures appear in sandstone due to the coupled effect of thermal expansion of quartz and feldspar particles and dehydration of attached clay minerals, resulting in an increase in the number of larger pores（T2>10 ms） and pore volume. When the coal seam is degassed by microwave heating, the coal seam should be sampled and analyzed first, and the degassing effect of the coal seam by microwave heat injection is determined by the content of carbonate, sulfide and moisture in the coal. In addition, in order to avoid the damage of roof rock caused by microwave heating in coal seam, microwave output with appropriate wavelength should be selected according to the extraction range of coalbed methane.