The continuous fog process from January 5 to 11 2008 in Shanghai is analyzed using remote sensing visibility and conventional observation, GFS and T213 model, and GPS PWV data. The results show when there are frequent synoptic scale troughs moving east and warm moist air flow in front of the troughs, it is easy to form radiation fog in sunny and breezing morning, in addition with warm moist air near the surface layer from warm water or sea surface. When warm moist air near the surface layer flows from the East China Sea or the Yellow Sea to cold water or land surface, it is also easy to form advection fog, in addition with the convergence of the low level southerly warm moist air from the south of the Yangtze. The range of advection fog from sea is closely related to the strength of easterly air flow. The stronger the easterly air flow is, the longer the advection fog would moves. The research also shows that the area with high average relative humidity within 0 to 300 m above the surface is consistent with the fog zone during the advection fog period. When absolute humidity is less than 3 g/kg, there would be no fog forming. While absolute humidity is between 9 and 15 g/kg and the GPS PWV is about 15 mm, the probability of thick fog would be high under appropriate weather condition.