Using conventional observations, satellite TBB, radar reflectivity, NCEP reanalysis data and unconventional observations including wind profile, automatic weather station and ground-based radiometer data, the characteristic of the occluded front of a backflow snowstorm in Beijing area on 9 November 2009 was studied. The result shows that: The slope of the front from north to south is gentle, the front in Beijing area is above 800 hPa, and the ∇θ_se and ∇T are serried vertically, the inversion (t/p) is strong. The warm moist air moves on the dry cold air which is the direct reason of the snowstorm. The lower level cold air becomes strong at the plain area because the block of the Taihang mountain, the intensity of the inversion can stand that of the front, the lower level Northeast airflow is dry and cold and it meets the warm moist air at the height of about 2000 m. The dynamical frontogenesis plays a very important role in the development of backflow snowstorm, the frontogenesis secondary circulation directly causes the snowstorm. There is unstable stratification in the front, the thunderstorm was triggered over the boundary level.