The western North Pacific is the foggiest oceanic area in the world, yet the synoptic processes of sea-fog formation over open oceans remain unclear due to the lack of observations so far. A Chinese arctic research vessel, Xiang Yang Hong 01, captured a fog event in the cold Oyashio Extension (OE) area from 12 to 14 September 2019. The formation and maintenance mechanism of the fog is investigated based on the in situ observations. The results show the fog is influenced both by a synoptic-scale warm front in an extratropical cyclone and a local sea surface temperature (SST) front. The warm moist air is transported from south to north along with the warm front, which results in the near surface air becoming warmer and wetter in the OE area. The fact that the moistening effect is greater than the heating one leads to the increase of relative humidity approaching saturation. The northward warmer air climbs upward when it meets the colder air mass over the OE waters, thus forming a large-scale frontal inversion. The forcing of the local SST front brings about a secondary circulation in the marine atmospheric boundary layer and the bottom of the inversion drops under the effect of a sinking branch from the circulation. The lowered inversion is conducive to the formation of fog droplets, which, once formed, are confined near the sea surface. The fog layer is formed in the warm moist air on the cold flank of the SST front. The contributions of the background atmospheric warm front and the local forced SST front to the sea-fog formation are clarified, which can provide a theoretical support for the forecast of sea fog.