Abstract: On the basis of the routine meteorological data and the reanalysis data from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) , the related circulation background, the meteorological fields and the physical quantity field of the foggy weather happened in Chengdu from January 31 to February 1, 2014 are studied. The results showed that this process was composed of radiation and advection fog, happened under the circulation of northwesterly flow in the 500 h Pa and uniform pressure field in the sea level pressure. The weak precipitation before the process, the water vapor from the south wind and the disturbed western wind provided the vapor conditions. The warm advection form the lower level, vapor convergence and the ascending motion were beneficial to the formation and the maintain of this process. The height and intensity of the inversion layer, the double inversion structure, the thickness of the intruding cold advection and the thickness of the humidity layer are closely related to the concentration of the fog.