The phase evolution and formation mechanism of two snowstorm processes in the middle and lower reaches of the Yellow River in late February 2021 are analyzed by using multi-source observation data including conventional high-altitude data, data of densified ground automatic stations, dual-polarization Doppler radar, and microwave radiometer as well as ERA5 reanalysis data. The results show that the large-scale synoptic systems of the two processes are basically the same, but the intensity and position of the systems are different, leading to some differences between the two processes. There are phase transitions in both processes, phase reversal (from rain to snow and then to rain again) occurs in Process Ⅰ, while only the transition from rain to snow occurs in Process Ⅱ. In mountainous areas west of Taihang Mountain, the precipitation phase is mainly snow when the temperature at 2 m is lower than 0.5 ℃, and snow or sleet coexist in the range between 0.5 ℃ and 1 ℃; while in plains, the precipitation phase is rain or sleet when the temperature at 2 m is between 1 ℃ and 2 ℃, snow and sleet coexist in the range between 0 ℃ and 1 ℃, and the precipitation phase is snow when the temperature is lower than 0 ℃. In the rainfall stage, the dual-polarization radar correlation coefficient (CC) is below 0.98, the differential reflectivity (Z_DR) is above 0.6 dB, and the specific differential phase shift (K_DP) is above 0.2 (°)·km^-1; while in the snowfall stage, CC is between 0.98 and 0.99, Z_DR is between 0.2 dB and 0.8 dB, and K_DP is below 0.2 (°)·km^-1; however, when the precipitation phase changes from rain to sleet, the values of Z_DR, CC, and K_DP do not change significantly.