Base on the ECMWF analysis and forecast data (025°×025°), wind profile data, and radiosonde data, the transformation process of raintosnow and then snowtorain in Jiaxing on 18 February 2014 is analyzed. The result shows that the temperature at 850 hPa and 925 hPa declined during the daytime, but the transformation from rain to snow occurred in the morning and that from snow to rain in the afternoon. In the morning, abundant water vapor was lifted to the higher and colder height, and then condensed, forming dense snowflakes; when snowflakes fell, the temperature at lower levels was not high enough for them to melt. While in the afternoon, water vapor mainly existed at the low levels, where temperature was only between -2 ℃ and -3 ℃. Supercooled water droplets instead of snowflakes formed in this environment, and at the same time surface temperature increased up to over 0 ℃ by the effects of warm advection near the ground from the sea, so the precipitation phase transformed from snow to rain.