In order to find out the evolution characteristics of successive heavy fog and haze processes and reveal crucial conditions for the formation and development of fogs in Beijing, this paper analyzes the weather evolution characteristics corresponding to the heavy fog and haze case occurred from 26 Jan to 31 Jan 2013. Conventional meteorological data, automatic weather station data, and atmospheric composition observation data are used in this study. The weather conditions that are favorable for the formation and development of fogs are also analyzed. Based on the above analysis, the complicated BJ-RUC v2.0 (Beijing Rapid Updated Cycle forecast system version 2.0) is used to simulate water vapor, momentum, and thermal conditions for fogs. The results indicate that BJ-RUCv2.0 can well simulate the area of advection fog that occurred from the night of 30 Jan to the early night of 31 Jan. However, large errors occurred in the simulation for the fog event accompanied with severe air pollution from the nighttime of 28 Jan to the daytime of 29 Jan. The crucial condition for the formation of the heavy fog is the increase in the atmospheric humidity below 950 hPa, which is attributed to successive southeasterly winds in the near surface layer. The atmospheric stratification stability and the inversion layer are strengthened by the obviously warm advection from 975 hPa to 800 hPa, which is an important factor for the development and maintenance of the heavy fog. In addition, the wind convergence near surface (below 950 hPa) and the wind divergence on the upper layer are favorable for the fog development.