Based on NCEP 1°×1° 6h interval reanalyzed data and daily observation data, the circulation background, synoptic weather systems and the formation mechanism of the cold wave event covering the whole country on 10—13 March 2005 are discussed. The results show that the formation mechanisms of the event are as follows. (1) The strong warm advection of 500hPa over the west Europe led to a blocking high created in Siberia area, and a zonal though, the north-east stream in the north edge of the though led to super polar cold air and the cold air from west together and got to strengthen, and then the abnormally strong cold high, front zone, cold temperature center and cold temperature advection appear. (2) The two short-wave troughs moving eastwards intruded into the blocking high, which made the zonal trough created and became a meridianal one for two times, resulting in the strong cold air greatly moved into southern area. (3) The three conditions, involving the temperature sharp drop, the convergence of west-south jet stream and the cold air stream at 700hPa, and the vertical temperature distribution of the area below 0℃ above 1000hPa, provided the main factors of dynamics, moisture and condensation for severer snow in southern China.