From 10 to 14 April 2022, a long-duration, wide-range and high-concentration sea fog event occurred in the coastal waters of Zhejiang. The characteristics and causes are analyzed by using satellite data, ECMWF (European Centre for Medium-Range Weather Forecasts) Reanalysis-Interim (ERA-Interim) data and data of coastal automatic meteorological observing stations in Zhejiang. The results show that the inversion layer makes it difficult for the low-level water vapor to spread to the upper air, which is conducive to the generation and maintenance of fog. In the fog formation stage, there is obvious water vapor convergence, and at the same time, the air temperature is higher than SST (sea surface temperature) and the difference is between 0 and 2.0 ℃. When the sea fog maintains and the concentration is high, the water vapor convergence gradually weakens, and the air-sea temperature difference (Δt=ta-ts) is around 0 ℃. In the dissipation stage, there is obvious water vapor divergence; the air-sea temperature difference is greater than 2.0 ℃ or less than 0 ℃.