Based on the diagnosis of moist potential vorticity (MPV) and the theory of slantwise vorticity development (SVD), a case study of snowstorm in the Tibetan Plateau induced mainly by Bay of Bengal storm in November 2007 was analyzed. The results show that the interaction of the storm′s spiral cloud band and the down\|sliding dry cold air (positive MPV1 band) over the steep southern fringe of the plateau is the main reason for this synoptic process. Owing to the dense and obvious slantwise isentropic surface, the contribution of negative MPV2 to moist potential vorticity (MPV) is more than MPV1 over the snowfall area where atmosphere is weak stable. It further leads to the sharp growth of SVD. Moreover, negative MPV makes a favorable background of the generation of conditional symmetrical instability. The conditional symmetrical instability is beneficial tothe development of slantwise vorticity, and it is a possible important mechanism for this snowstorm.