An advection fog event occurred along the coast of Qingdao from 22 to 25 May 2024. This study analyzes the microphysical characteristics of the sea fog using the data of FM-120 fog monitors at two coastal observing sites (Baguan Hill and Xiaomai Island), observations of surface automatic meteorological stations and ERA5 reanalysis data. The results are as follows. (1) The fog event is characterized by stable synoptic situation, southeast warm and moist airflow and a strong inversion layer. (2) During the event, the average number concentration at Baguan Hill (Xiaomai Island) is 277.02 cm-3 (77.58 ^cm-3) with an average liquid water content of 0.051 g·^m-3 (0.027 g·^m-3), an average effective radius of 4.92 μm (7.39 μm) and an average radius of 2.65 μm (2.78 μm). (3) The microphysical characteristics from these two observing sites exhibit synchronous temporal evolution, and the fog droplet spectrum displays a distinct bimodal structure during the development and maturation of the fog, particularly evident in the large particle range above 15 μm. (4) The visibility parameterization scheme established based on both number concentration and liquid water content shows optimal predictive performance during the fog event.