A heavy fog event with visibility less than 1000 meters covering most parts of Hinggan League on 21 and 22 October 2013 is analyzed. The conventional data on the MICAPS system, radiosonde data, and NCEP 25° × 25° reanalysis data are used to diagnose and analyze this fog process. The results show that: the foggy weather process lasted two days. The strongest fog occurred in the day time of 21 and 22 October with the minimum visibility less than 100 meters. The fog occursred in the southwest airflow at 500 hPa and the warm ridge at 850 hPa, and the forehead of a lowpressure field and a weak high pressure field are manifested on the ground. A stable existence of an inversion layer and southeast warm transportation provided favorable conditions for fog maintenance. The decreasing of ground humidity caused by the strong ascending air in the front of the highaltitude cold vortex is the main reason for the dissipation of the fog.