The characteristics and forming, maintaining and dissipating mechanisms of a sustained sea heavy fog occurred over the Yellow Sea, Bohai Sea, and adjoining areas from 31 May to 5 June 2010 is analyzed by means of various data, including MTSAT1R satellite data, conventional surface and upper air observation data, FNL (objectively reanalyzed Final Analysis） data issued by NCEP and daily averaged sea surface temperature (SST) from NEARGOOS (NorthEast Asian Regional Global Ocean Observing System). The results indicate: the moisture in the lowlevel air was very abundant before the fog, and a transformed cold high pressure sustained stably over the Yellow Sea and Bohai Sea and provided a good background condition for the sustained sea fog, and then sea fog formed mainly in nighttime because of radiation cooling. During the fog, air temperature was just a little higher than SST, and subsidence inversion existed in the lowlevel air below 975 hPa. The development of cold temperature advection also contributed to the cooling of moist air in the lowlevel air, which played important roles in the development and sustaining of the sea fog. As air temperature rose again and the moisture dropped, relative humidity descended gradually and led the fog to disappear slowly.