On the basis of Analysis of the synoptic situation and the visibility for the heavy fog event on 26 October 2007 in Beijing area, comparative analysis of the fog area, the boundary layer distribution characteristics of the temperature, humidity, wind speed, and liquid water content (LWC) is conducted with the observation data of the radiometer, wind profiler, satellite, and the simulation results. In addition, the mechanism of the fog formation, maintenance and dispersal is analyzed with the simulation results of a mesoscale model. The results indicate that the long-wave radiation cooling is the important factor of the fog formation in addition to the steady atmospheric stratification and plenty of moisture, while the fog dispersal was most influenced by the shortwave radiation.