The satellite, radar and NCEP data are used to analyze an unusual snowstorm over Liaoning Province during 3-5 March 2007. The analysis shows that the combination of south and north troughs at 500 hPa, the northbound vortex at 850 hPa and the cyclone at the surface are very favorable to the occurrence of snowstorm. The intense low level jets and ascending motion at 850 hPa are in favor of the moisture transportation and the intensification of snow. The low level jets and the convergence of wind may be considered as triggering mechanism for the snowstorm.