In order to study the formation mechanism of urban heavy fog and the pollution o f fogwater, an observation of a heavy fog with the visibility of 100m during 1 1-12 Dec, 2006 was made in Beijing Observatory located in the south suburb (39° 56′N，116°17′E). The synoptic system and fogwater ionic species concentrati on was analyzed in the work. The results demonstrated that the observed fog proc ess was an advectionradiation fog with a deep layer of more than 150 meters and a low visibility of less than 100 meters. In the front of upper t rough there exits a stable boundary layer, accompanying with convergence and wa rm moist advection in the surface positive relative vorticity area. The surface temperature decreased rapidly after sunset, which is favorable for the fog f ormation. The southoriented wind and warm advection layer on the ground result in an increase of fog concentration. The formation and increase of heavy fog acc ompany with the air pollution index Ⅴ(APIⅤ) increasing. Compared with Beijing' s fogwater sample of 1999, the pH rises, electric conductivity rate descents, t he anion’s concentration generally decreased and the cation concentration incre ased. Investigation shows that the pollution of heavy fog and fogwater of Beij ing has been reduced through efficient control.