In order to build the Objective Forecast System of Heavy Fog for Sichuan Basin, an analysis is made of the formation causes of heavy fogs over the Sichuan Basin by means of both synoptic and statistic methods. The results show that, fogs occur much more frequent in the Sichuan Basin than in other places, which is closely related to high humidity in the surface layer resulted from the geographic surroundings. The contributors to heavy fogs in the Sichuan Basin include stratification stability, weak wind, and quasi saturation in the surface layer. The key step to forecast heavy fogs is to determine the occurrence probability of rainfall and the intensity of radiation cooling.