Using conventional meteorological observations, NCEP reanalysis data (FNL), L band radar data, and applying the multi channel satellite monitoring data from National Satellite Meteorological Centre and qualitatively analyzing the sea fog, this paper discusses the heavy sea fog event during February 22-25, 2010, including the boundary layer (BL) characteristics, formation and disappearance mechanism. The results show that: (1) The fog happens under straight circulation in the Eurasian high latitudes, with stable atmospheric stratification weather conditions. The southwest flow of southern branch trough converges with the southerly flow in the northwest of subtropical high and behind high pressure ridge, and the corresponding low latitude southerly flow provides a favorable moisture condition for the formation of this event. (2) The process of fog is closely related to sea surface temperature, air sea temperature difference, stability of atmosphere, wind and other meteorological or hydrological factors. During the fog process, air sea temperature difference is 0-2℃, inversion and isothermal layers appear from BL to the lower troposphere. There is a 6-8℃ in inversion layer, and vertical temperature increasing by time is greater in the upper level than in the bottom, which causes the inversion layer to strengthen and continue to uplift or reconstruct, and restrains the vertical convection. Air humidity is comparatively large in the near surface layer with a center of 0.6 g·kg-1 in liquid water content around 200 m. And there is 2-4 m·s-1 northeasterly clockwise turning to 6-8 m·s-1 southwesterly below 850 hPa, which provides favorable conditions for the fog. (3) The developed height of turbulence reaches 240 m. The turbulent mixing brings moisture and droplets from upper layer to the surface layer, and contributes to the cooling, which results in the saturation and condensation for the fog event. The warm air in the lower layer is transported to the cold sea surface by the sustained weak and warm advection, which is conducive to the establishment of the inversion. The radiation cooling triggers this fog event.