Based on the conventional meteorological data, observation data of the Tianjin Observation Tower, radar data, VDRAS data (Variation Doppler Radar Analysis System), and NCEP reanalysis data (ECWMF and National Centre of Environmental Prediction), the causes of the first snow weather process on 20 and 21 November 2016 in Tianjin are analyzed. The results show: the mixed rainsnow storm was produced in the interaction of the upper trough and returnflow cold air; the main sources of water vapor were the southwest warm and humid airflow in front of the trough in the middle and lower troposphere and the returning east winds. The returning east wind absorbed moisture when it passed through the low surface of the Bohai Sea and became wet and cold; the dynamic condition was mainly from the dynamic lifting of the cold return flow, the thickness of the returning easterly increased from 1.5 km to 2 km during precipitation. The nongeostrophic secondary circulation transported the returning water vapor upwards into a source for precipitation, and strengthened the vertical ascending motion of the warm and humid air to provide dynamic conditions. The north wind component wind speed and thickness of the cold air increased sharply. Simultaneously, the conversion of highaltitude cloud water particles to cloud ice particles and the reflow of cold air at the boundary layer were indispensable for the transformation from rain to snow, and the uniform temperature layer and cloud ice particles of 800 to 950 hPa increased greatly and extended to low altitudes. The variation characteristics of the potential thickness of 700 to 850 and 850 to 1000 hPa had good indication value for the discrimination of rain and snow phases.