Based on the routine observation data, T213 products, and FY2C water vapor images, the circulation background, the influencing synoptic systems, and the causes of the extraordinarily serious snowstorm event occurred from 3 to 5 March 2007 in Liaoning Province are analyzed diagnostically．The results show that （1）The extreme weather event happened under the background with the circulation index being high and the most part of China being warmer,associated with gale and intense temperature decrease.（2）The Jianghuai cyclone was the most important influencing weather system; the occurrence and development of the Jianghuai cyclone were resulted from the joint effects of positive vorticity advection at high levels and warm advection at low levels in the troposphere.（3）The strong ascending motion caused by the vertical allocation of the divergence at upper levels and the convergence at lower levels is the dynamical formation and maintenance mechanism of the snowstorm. (4) The cyclone occurred in the area between polarfront and subtropical westerly jets, moved from the north of the exit area of the subtropical westerly jet to the south of the entrance area of the polarfront jet, developed intensely during the process moving from the left side of the southwest jet stream in the lower levels to the left front.（5） It is found that a dry intrusion is the important feature of intense development of the cyclone. (6) The southwest and southeast jets at the lower levels offered plentiful water moisture for the heavy storm (rain), which occurred within the warm and moist tongue at 850 hPa, whose precipitation is directly related to the temperature at 850 hPa. (7) The intense temperature decrease happened under the conditions of the development and eastward movement of the longwave trough leading to the southward movement of polar cold air, and the strong winds were resulted from the tight pressure gradient between intensely developed cyclone and strong cold high.