Diagnostic analysis is performed to study the snow cover characteristics of an extreme rain-snow event over Shandong from 6 to 8 November 2021 by using data of automatic weather stations, artificial densified hourly snow depth observations, and ERA5 reanalysis data. Main results are as follows. (1) The rain-snow process is an extreme weather event for its precipitation higher than the extremum of the same period in history, and snow depth poses a challenge to forecasters. (2) Snowstorm and snow cover area are concentrated in the central and northern parts of Shandong, the range of quantitative snow cover is basically consistent with that of snowfall R≥5 mm, and the snow depth has obvious temporal variations. (3) The extreme rain-snow event is caused by cold air under favorable water vapor and dynamic conditions and typical backflow snowstorm weather situation. The low-level strong cold advection leads to the precipitation phase transformation, and snowstorm and severe snow cover occur in the central and northern parts of Shandong. (4) The snow-to-liquid ratio differs greatly in snow-covered area and the mean value is 0.53 cm·mm^-1, which is lower than the historical average. Snow depth is related to the configuration of temperature, relative humidity, and vertical velocity in the upper level; low temperature is conducive to snowfall and snow cover. The geographical location of Shandong, mountainous topography of central Shandong, and surface wind speed have influence on snow depth. The sea-land difference has greater influence on snow depth than latitude difference does, while altitude has less influence. (5) The snow cover products of ECMWF weather model perform better in forecasting the snow cover in Shandong with small errors, though the forecast is overall weaker in that the snow cover is less in the northern part and more in the central-southern part than the observational data.