Based on conventional meteorological observation data in Tianjin and the ECWMF ERAInterim reanalysis data (0.125°×0.125°), the transformation processes of raintosleet and then sleettosnow were analyzed from October to next April during 2000-2015 in Tianjin. The humidity and geopotential thickness during the transformation process of precipitation phases were analyzed, and 9 factors, namely temperature on 850 hPa, 925 hPa,1000 hPa and surface, thickness from 1000 to 850 hPa and from 850 to 700 hPa, 0 ℃ layer height, -4 ℃ layer height and 925 hPa relative humidity, were identified, which had close relationship with transformation process of precipitation phases. Further, their corresponding threshold indexes for different precipitation phases were given. And through the analysis of 3 weather cases, the usability of the indicators had been further verified. The discriminant equation for precipitation phases was established by using the 9 factors, it was found that the backtesting discriminant accuracy for rain and snow was higher than 80%.