By using symmetry instability criterion,an unusual storm smow appeared on January 6 to 7,2001 in Shandong Province is analysed,the results show that the storm snow occurred in the symmetry instability atmosphere,low-level jet caused warm and humid air converging and ascending in lower troposphere and stimulated the symmetry instability energy releasing and then the event of storm snowfall occurred.The Changjiang-Huaihe cyclone and the region of storm snow moved towards the area of symmetry instability.