Abstract: From January 6 to 8, 2021, there was a cooling weather in Chengdu Plain, and there was a wide range of rain and snow from the night of January 6 to the night of January 7. Based on conventional observation data, ground data and wind profile data, this paper analyzes the causes of cooling and snowfall weather and the transformation of rain and snow phase. The main conclusion are reached as follows: The circulation pattern of the cooling process was Central Asia transverse trough. The path of the cold air was northwest. The convergence of warm and cold air in the middle and lower layers in Sichuan Basin brought continuous precipitation weather to Sichuan, and the continuous precipitation had an increasing effect on the temperature drop. When the temperature of 850 hPa was lower than −5℃, the surface temperature was lower than 2℃, and the height of 0℃ layer was lower than 920 hPa, it's beneficial for precipitation phase to change to snow in Chengdu Plain.