Abstract: For exploring the causes of sea fog over the South China Sea in boreal spring and the forecast capability of a numerical model, the observational data of atmospheric boundary layer are employed, and the formation and development processes of a sea fog event near the Pearl River Mouth in boreal spring between 21 and 22 March 2006 are analyzed. Then the 3-dimensional numerical simulation is carried out on the process of this sea fog by using the Weather Research and Forecasting (WRF) mesoscale model. The observational results show that this sea fog belongs to an advection cooling fog. It was caused by the cold air from north moving into the South China Sea and then weakening and disappearing when the north of the South China Sea was influenced by the southern warmwet air flow with the air temperature warming up rapidly. And it was a result of interaction between cold sea surface and warmwet air. The simulation results show that the forming time and process of developing evolution of this sea fog are very close to the observational result. The spatial change of this sea fog, the atmospheric stratification change of boundary layer, the change of surface sensible heat flux and the formation cause of this sea fog simulated by the model are all relatively consistent with the observational fact. And the numerical model shows a good simulation skill and ability on the process of this sea fog. This sea fog's formation and development mainly have some connection with the cold underlying surface, the influence of warm and wet air, and the stable atmospheric stratification near ground.