Based on the conventional observation and NCEP reanalysis data, the synoptic ana lysis of a rare heavy fog process occurring in the eastern China was carried out. The result shows that the height and intensity of the inversion layer are related to the formati o n and status of the fog, weak cold and warm advections are both advantageous to pr oducing fog. Furthermore, the temperature advection quickly turns into reverse at some height near the surface, and thus makes temper ature stratification from unstable into stable, all these are benefical to prod uce the hea vy fog. The inversion layer which formed in the interface of ascending and desce nding movement is the important fact for producing the heavy fog.