The spring Yellow Sea fog event with 2 large patches on 14 April 2012 is chosen for study. Using the WRF (Weather Research and Forecasting) model and cycling-3DVAR (3-Dimensional Variational) data assimilation (DA) scheme, combining with the background error covariance CV6 of the humidity control variable, DA experiments of AIRS (Atmospheric Infrared Sounder) air temperature and humidity profile data are carried out. The formation mechanism of the sea fog is investigated based on the results of the DA experiments. The DA experiments indicate that the DA of AIRS data can successfully reproduce the sea fog event, especially the clear sky region between the eastern and western fog areas. This result is contributed by the improvements of the structures of temperature and humidity in the marine atmospheric boundary layer, as well as the range and intensity of the low-level high-pressure system. The mechanism analysis reveals that both the eastern and western fog patches belong to typical advection cooling fogs while their thickness and air mass original breeding sources are different. The warm air over the land to the west coast of the Yellow Sea is moved over the sea surface by the high-pressure system, and an inversion layer is formed by the cooling effect of colder sea surface temperature (SST), then at its bottom the thin western fog patch occurs. The air from the central Yellow Sea moves northeastward over the sea region west to the Korean peninsula, then high-pressure subsidence results in a deep stable layer and the thick eastern fog patch occurs. Within the high-pressure region, near the sea surface weak winds make weak mechanical turbulence, air-sea temperature is small due to the joint action by warmer air by subsidence and SST warm tongue. Thus sea fog forming is suppressed and a clear sky region appears over the central Yellow Sea.