Based on automatic weather station, dual-polarization radar, millimeter-wave cloud radar, temperature profiler and ERA5 hourly reanalysis data, the characteristics of the multi-source observation data of the precipitation phase variation during the extremely heavy snowstorm in the western area of Shandong on 7 November 2021 are analyzed. The results show that this process was commonly influenced by the westerly trough, low vortex shear line and cold front. The system had strong baroclinicity, with low-level jet. Though the snowfall duration was not long, the hourly intensity of snow precipitation was high, resulting in extremely heavy snowfall in the western part of Shandong. The multi-source observational data captured the precipitation phase and change information well in this process. Wind and vertical velocity from wind profiler radar, correlation coefficient and 0℃ layer bright band of dual-polarization radar, reflectivity, vertical liquid water content, velocity and spectral width of millimeter-wave cloud radar, boundary layer temperature of temperature profiler and the two-dimensional images and information of particles from the 2D video raindrop spectrometer were used as indicators for judging the phase of the precipitation, so these information provided a reference for the 0-1 h short-term nowcasting. During this process, air temperature above the ground cooled very rapidly, the rain quickly turned to snow with the sleet only lasting for a short time. The cooling of air temperature above the ground was caused by the combination of strong negative temperature advection, vertical motion and non-adiabatic effect, of which, the effect of negative temperature advection was the largest, followed by precipitation diabatic effect, and the vertical motion effect in order. The strong negative temperature advection and the negative temperature advection intensity increasing with the decrease of height, led to the rapid cooling of air temperature above the ground.