Based on the data from conventional observations and NCEP reanalysis, a synoptic analysis was carried out for the boundary of a rare large fog process occurring in the early November, 2008. The result shows that, at the boundary of the fog field, besides such basic conditions as temperature inversion， air humidity an d wind speed,the divergence field and vertical wind speed are related to the fog formation, and the boundaries of lowlevel convergence and highlevel diverge nce are also related to the fog boundary.