Based on the data of meteorological observation stations and Doppler weather radars, lighting data, artificial intensive snow depth observations, conventional observations, and ECMWF (European Centre for Medium-Range Weather Forecasts) Reanalysis v5 (ERA5), the characteristics and causes of the extremely severe sea-effect snowstorm over Shandong Peninsula from 15 to 22 December 2023 are analyzed. The main results are as follows. (1) The snow depth of 4 stations exceeds historical records and the daily snowfall of one station in 2 d tops the records of sea-effect snowfall over Shandong Peninsula. The snow depth of 74 cm in Wendeng exceeds the records of all meteorological observation stations in Shandong, so it is an extreme sea-effect snowstorm event. (2) The two abnormally strong cold air processes under the blocked circulation in middle and high latitudes over Eurasia continuously affect the Bohai Sea and Shandong Peninsula, and the temperature of 850 hPa drops to the range between -21 and -20 ℃. The cold air is significantly stronger than that of the sea-effect snowstorms in December of previous years, resulting in a long duration of snowfall and a large amount of accumulated snowfall. The abnormally strong cold air is the fundamental reason for the extreme snowfall process, and the abnormally high SST (sea surface temperature) over the Bohai Sea is a favorable background. (3) The strong cold air and higher SST lead to larger air-sea temperature difference and unstable convection below 700 hPa, which causes high snowfall intensity, and the heavy snowfall occurs in the stage when the air-sea temperature difference increases rapidly. (4) There is a shear line composed of northerly-northeasterly from the Bohai Sea and northwesterly from inland areas below 925 hPa, which produces a strong ascending motion, and the shear line’s maintaining contributes to the formation of train effect. (5) During the main snowfall period, the temperature in the layer of strong vertical ascending motion and high relative humidity lies between -20 and -12 ℃, which is suitable for the formation and maintenance of dendritic ice crystals and conducive to the generation of large snow cover and snow-to-liquid ratio. The air temperature at 2 m is continuously lower than -5 ℃, the ground temperature at 0 cm has dropped below 0 ℃ since the beginning of snowfall, and the interval between the two heavy snowfall processes is only 1 d, which are all conducive to the extreme snow depth.