At the beginning of 2008，Hunan province experienced an extreme freeze catastrop he that is rare since the complete meteorological record has been set. Analysis with conventional data indicates that the stable block high in the middle latitu de at high level of atmosphere, sustained strong subtropical high pressure, and stable Bengal trough kept Hunan in the southwest stream of the leading area of t rough. Warm and wet air mass brought by southwest jet at 700hPa sustained the pe rsistent precipitation and it was favorable for the temperature inversion. The s hear line at 850hPa oscillated within Hunan, the surface cold air invaded southw ards from Lake Dongting and led to large area precipitation. The macroscale circulation provided favori te background for the freeze disaster. Research on the relation among the air ve rtical structure, surface meteorological elements and freeze shows that the laye rs of inversion, melting layer and the surface temperature below 0℃ are key fac tors to form the freeze, the liquid precipitation played an important role, and the daily rainfall above 0.1mm was favorable for the formation and development o f freeze. The property of precipitation is an important factor. The persistent, large scale and stable drizzle and sleet are beneficial to the formation and dev elopment of freeze. Air temperature shows negative correlation with freeze devel opment. The temperature is the crucial for freeze when the vertical structure, p recipitation and geographic height are similar.