Abstract: An extratropical cyclone across the Yellow Sea caused a large-scale sea fog event from March 27 to 28, 2021. To investigate this event, we used a comprehensive set of tools, including ERA5 reanalysis data with a spatial resolution of 0.25°×0.25°, ground observation data from the China Meteorological Administration, satellite retrieval data, and the WRF model. Our results were as follows. (1) In the initial stage of this sea fog event, the difference between air and sea temperatures ranged between 0.5°C and 2°C. However, during its mature stage, this difference turned negative. (2) The moist air convergence driven by the extratropical cyclone served as the primary source of water vapor, which was essential for the formation and maintenance of the sea fog. Furthermore, the southwest warm and moist air flow, transported by the low-level jet, facilitated the formation of the inversion layer. (3) During this event, an interesting phenomenon was the appearance of a “double inversion” in the Yellow Sea. The inversion layer in the low level was caused by the warm advection of the low-level jet, while the higher-level inversion resulted from the sinking and warming of the dry and cold air behind the extratropical cyclone. (4) The combination of the extratropical cyclone and the low-level jet led to an increase in atmospheric turbulent kinetic energy within the near-sea surface boundary layer. This synergistic effect further promoted the vertical development of the sea fog. (5) The diagnostic results of turbulent kinetic energy budget showed that shear production played a dominant role in the occurrence and development of sea fog. When sea fog developed in the vertical direction, the vertical transport of turbulent kinetic energy increased significantly.