Based on the observation data ,the analysis of the regional snowstorm event occurred in south of Huaihe river during 27—28 January 2008 was made. The results showed that the background conditions of snowstorm is the combination of cold air supplemented continuously on the east side of blocking high located at Balchas Lake with low level southwest jet stream transportation of warm, moist air at the low level. The main influence systems are cyclonic circulation and shear line . Through analyzing numerical simulation data,the results showed: moisture flux divergence and vertical motion revealed that the middle atmosphere above 700 hPa is the key; atmospheric temperature of the whole atmosphere below 0 ℃ is the sufficient conditions.Through diagnostic analysis of ζp and ζm the results showed : cold air wedging from north to south in the lower layer caused that warm and wet flow inclined upward to this cold cushion. Slantwise updraft caused symmetric instability which developed vertical motion. Mesoscale symmetric instability is the physical mechanisms of heavy snow. 