Based on the NCEP data (1°×1°) and conventional observation data, a utomatic meteorological observing station and Doppler radar data, the forecast o f the transformation between the rain and the snow as well as the cause of phase transformation in precipitation in one of strong cold waves happened during the early spring in Hebei Province is analyzed. As a result, the easterly wind brou ght by the Mongolia high and the JiangHuai cyclones below 850hPa make the temp erature of the lower atmosphere decrease, when the height of the 0℃ atmospheric layer is lower than that of the 950hPa, the character of precipitation will cha nge from rain to snow. Also when the wind directions at both the high and low levels turn from northeast to north west along with the JiangHuai cyclones moving into the sea and the temperature at the low level around the east of Taihang Mountain becomes rising slowly with the 0℃ atmospheric layer going upper, the snow will turn to be rainfall. According to the echo characteristics of Doppler radar , when the height of the 0℃ layer bright band decreases quickly, the snow will change into rainfall. While the temperature of the 1000 hPa layer goes down to 2 ℃ and the temperature of the 925 hPa layer goes down to -2℃, the precipitation will change from rain to snow.