Based on conventional observations and NCEP 1°×1° reanalysis data, causes and precipitation type of the rain and snow weather process in North China during 3-4 November 2012 are analyzed. The results show that the deep low vortex and surface cyclone are the systems that directly impact the process. The southeast low level jet transporting the abundant moisture from the eastern sea leads to the atmospheric precipitable water over the entire rain and snow storm area, exceeding the average of the month. The high level divergence systems superimposed on the low level convergence systems of the vortex and the cyclone provide a strong and lasting upward movement for the severe rain and snow storm. The heavy rain area and the blizzard region respectively stand for the convection instability stratification and the conditional symmetric instability stratification, and the frontogenesis favours the heavy snow storm. The environmental conditions of the snowflake formation and growth, and the melting of the snowflakes falling are equally important in determining the precipitation type. The combinations of the two reasons mentioned above ensure the emergence of snow on the ground surface.