During the Spring Festival of 2018, there was a sustained fog process lasting for 6 days (from 15 to 20 February) over Qiongzhou Strait, during which the minimum visibility was less than 200 m. This paper mainly analyzes the temporal evolution of the sustained sea fog process over Qiongzhou Strait by satellite monitoring and reveals the reasons for its persistence based on atmosphere and ocean conditions. Firstly, the SST in western Pacific on the east of South China is higher than that in normal years with three higher sea surface temperature anomaly (SSTA) centers in February 2018, which provides warm and moist air for the sustained heavy fog. Secondly, there is a static and even pressure field around Qiongzhou Strait and the southwest vortex plays an auxiliary role. The oceanic southerly winds resists the dry continental air mass at the boundary layer, which is in favor of maintaining the humidity of the boundary layer. Thirdly, the bottom layer is warm and humid; at the same time, a dry center maintains above the near-surface wet layer at 925 hPa. Such stratification is conducive to the maintenance and formation of sea fog with certain thickness. Fourthly, the increase of low clouds and weak precipitation are important factors for the maintenance of sea fog throughout the day on 18 February.