Compound extremely catastrophic weather events occur frequently in the background of global warming, and the frequent snowstorm or frozen rain event in southern China is a typical compound extreme weather event. Therefore, this study (1) analyzed the circulation characteristics and the rainfall phases of the heavy snow and freezing rain in a typical compound weather event in Southwest China, and revealed the characteristics of the two rain phases. It is found that the freezing rain occurring in the frontal strong baroclinic environment in Guizhou Province, but the snowfall occurred in the cold zone to the north of the front in Sichuan Province. Moreover, there is a significant difference in vertical circulation for that the ascent motions are over snowfall areas from low level to high level, whereas there is a temperature inversion layer in the lower atmosphere over the freezing rain area, which leads to a cold-warm-cold temperature configuration. Thus the vertical motions present a two-layer circulation mode, in which the strong ascent motions at low levels are inhibited by the descending motions at medium levels. The ascending motions are stronger over the snowfall area and the cloud top temperature is colder accompanied with the development of convection, resulting in the phase difference from the freezing rain. (2) Utilizing the generalized moist potential vorticity (GMPV) theory, which can comprehensively depict the circulation characteristics and the phase transition of water vapor, to diagnose the characteristics of the GMPV distribution during the occurrence and development of the heavy snow and the freezing rain, respectively. It is found that the anomalous distribution of baroclinic term can better reflect the atmospheric baroclinicity nearby the quasi-stationary front, as well as the location or evolution of heavy snow and freezing rain. Thus it can be regarded as one of the dynamic identification characteristics of heavy snow and freezing rain. (3) The results by calculating of the disturbed pressure equation terms show that the balance diversity between the downward perturbance pressure gradient force and the buoyancy is the main reason for the difference of the vertical circulation characteristics between snowfall and freezing rain. This study can provide references for weather analysis and operational forecast on the perspective of circulation characteristics of this type of weather，and provide reference for power operation guarantee of power generation enterprises. Key Words：Southwest China，snow and freezing rain，generalized moist potential vorticity， difference of circulation characteristics，precipitation phase