Based on the WRF (Weather Research and Forecast) model and T639 data as the initial and boundary conditions, a fog process in Jinan from 13 to 14 in November 2015 is simulated. The result of simulation test is verified by the routine meteorological observation data, and the causation of the fog is analyzed from four respects. The results show: (1) the stable atmospheric layer, gentle wind, small difference between temperature and dew point are the favorite conditions for the formation of fog; (2) the occurrence and development of the fog as well as the intensity of visibility can be simulated by the WRF model; (3) the maintenance of the neutral layer and enough moisture play the key part for the formation and development of the fog; the low clouds have effect on the formation of the neutral layers near the ground and postpones the dissipation of the fog in the daytime; (4) during the formation and development of the fog, the convergence ascending motion below 925 hPa and divergence sinking motion above 925 hPa can be favorable to the formation of the fog by making the moisture accumulate in the surface layers; (5) the cold advection can be beneficial to the fog formation by increasing relative humidity, while the advection below 850 hPa get warmer in the dissipation phase of the fog, the nonadiabatic factor have little impact on the formation and development of the fog.