This study investigates a persistent sea fog event over the Yellow and Bohai Seas from April 12 to 15, 2024, utilizing ERA5 reanalysis data, multi-source observations data, alongside the HYSPLIT model. The results indicate that the sea fog process can be divided into two distinct phases: the high-pressure rear advection fog phase (April 12th early morning-14th daytime) and post-cyclone frontal fog phase (April 14th nighttime-15th daytime). Significant differences exist in the formation mechanisms and characteristic physical parameters of the sea fog during these phases. The high-pressure rear advection fog mainly occurs in the central-western Bohai Sea and Yellow Sea, a persistent southerly flow continuously transported moisture from south to north, with both cooling and moistening effects working in conjunction, along with a strong inversion layer over the Yellow and Bohai seas, the sea fog exhibits extensive spatial coverage and prolonged temporal duration. The intense fog with visibility consistently less than 200 m for 38 hours occurs in the Yellow Sea, with the lowest visibility reaching 85m. Meanwhile, there is a significant diurnal variation in fog in some areas of the Bohai Sea, with visibility exceeding 200 meters. During the post-cyclone frontal fog phase, warm and humid southwest airflow prevails initially, and then the surface low pressure develops into a temperate cyclone. The cold air mass at the lower level penetrates and moves southward with the eastward movement of the cyclone. This cooling effect leads the air to reach saturation, accompanied with a stable stratification featuring a double-layer temperature inversion and an appropriate air-sea temperature difference, triggers persistent intense sea fog formation over the Bohai Sea and the northern Yellow Sea behind the cyclone. During this phase, the inversion layer intensity is weaker, and the duration of dense fog with visibility below 200 meters is only 7 hours, with the lowest visibility recorded at 71 meters. Furthermore, the two fog phases share some common features. The air-sea temperature difference is favorable, maintained 0 ~ 3°C in the Yellow and Bohai Sea. The high-value areas of 1000 hPa relative humidity (greater than 95%) demonstrates excellent agreement with the sea fog coverage in both geographical distribution and shape.