Abstract: The concept of wet bulb Potential vorticity and SCAPE (slantwise convective available potential energy) was used to calculate the horizonal and vertical distribution of moist symmetric instabillty including convective symmetric instability (CSI) and local symmetric instability (LSI) in the Process of a storm snowfall in North China.The potential contribution of moist symmetric instability to a narrow storm snowfall belt occuring in the Hetao and Lindong,Linxi region of Inner Mongolia and the relationship between moist symmetric instability and the direction of basic flow,wind shear and moisture were discussed.It was found that the strong snowfall belt is almost parallel with negative value area of wet bulb potential vorticity at low level in the vinicity of snow area.the dynamical mechanism of the strong snowfall centre in the Lindong,Linxi region is different from that in the Hetao region,the former is induced by frontgenetic forcing with weak symmetric instabilities in the warm section of frontal area;the latter is triggered by obious moist symmetric instabilities.