A 3D fog model was applied to investigate the genesis, development and major features of wintertime local circulation over Chongqing surrounded by mountains. Evidence suggests that downslope cold air pools the center of the city at nocturnal hours, changing to a valley-wind circulation around the noon. The circulation structure plays an important role in the genesis/dispersal of fog typical of the city. A numerical experiment was excuted concerning the impact of the circulation of fog, mountains and initial stability of the atmosphere.