FY-3D/MERSI-II polar orbiting meteorological satellite has 250 m resolution in 10.8 and 12 μm wavelengths of far-infrared channels, which provide more abundant data sources for fire monitoring. The characteristics of fire monitoring in FY-3D/MERSI-II 10.8 μm far-infrared channel were studied. Although 10.8μm far-infrared channel is far less sensitive than FY-3D/MERSI-II"s 3.8 μm mid-infrared channel in the spectral aspect to high-temperature heat source detection, its spatial resolution is 4 times higher than that of 1 km mid-infrared channel, so it can clearly reflect larger fire points, and its fire detection ability is better than that of 1 km resolution far-infrared channel. There has been a marked improvement. Using mixed pixel linear spectral separation method, the brightness temperature increment caused by the open fire area with an average temperature of 750 K and an area of 400 m² in far infrared channel pixels at 1 km resolution is about 0.47 K, while that caused by the same fire condition in far infrared channel pixels at 250 m resolution is about 7.30 K, which is quite different from the brightness temperature of surrounding background pixels. The fire location accuracy can be improved from kilometer level to hundreds meter level by using 1 km resolution mid-infrared channel to identify the fire range and 250 m resolution far-infrared channel to further determine the open fire area. Based on the method proposed in this paper, an example of forest and grassland fire application is analyzed. The location of fire point determined by 250 m resolution far-infrared channel is in good agreement with the field investigation information, which shows the effectiveness of the proposed method. Several application examples showed that the FY-3D/MERSI far-infrared channel has obvious advantages over the mid-infrared channel in reflecting the location of the intense fire area and the distribution of the open fire line of grassland fire in large-scale fire field. It can reflect the spatial distribution of wildfire more accurately, so it has application value in fire monitoring.