Using China’s firstgeneration global atmospheric/land surface reanalysis product (CRA), combined with visibility data from the CMA Land Data Assimilation System (CLDAS), automatic weather station and Himawari8 satellite data, the evolution, circulation pattern and boundary layer characteristics of the sustained sea fog process in Qiongzhou Strait from January 21 to 26, 2021 are analyzed, and also the formation mechanisms of two different types of fogs are discussed. The results show that: (1) the fog from January 21 to 22 was the frontal fog formed by the diffusion of cold air in the lower layer, while it turned into the easterly advection fog at the rear of a could high from 23 to 26. (2) During the frontal fog period, influenced by the northerly wind, the wind speed was 1 to 3 m·s^-1. During the advection fog period, the wind speed was 4 to 6 m·s^-1 due to the influence of easterly wind. (3) In the frontal fog stage, the water vapour convergence centre was located from the South bank of the Qiongzhou Strait to the northeast land of Hainan Island, and the fog began to develop on the land. In the advection fog stage, the water vapour convergence centre was located from the North bank of the Qiongzhou Strait to the east of Hainan Island, and the fog developed from the sea. (4) In the frontal fog stage, the inversion layer developed at the height of about 950 hPa, which was an advection configuration of cold in the lower part and warm in the upper part. In the advection fog stage, warm advection occurred below 950 hPa, and the inversion layer developed from the ground. The development of two kinds of fogs with different properties, frontal fog and advection fog, maintained the fog for a long time.