A system of terrain+following coordinates is developed to investigate a 3D flow-field over the complicated underlying surface,and thereby an objective analysis is done of the structure of the Chongqing urban area local circulation on a winter fine day. Results showthat downslope cold air from the surrounding hills flows into the urban region at nighttime hours and changes gradually into a valley wind circulatlon in the afternoon. This local circula-tion plays an important part in the fog generation/dispersion and pollutans distribution ofthe citvy.