Through the dynamical analysis on the sudden calamitous weather,it shows that:the sudden calamitous weather has the feature of quickly excitative rapid process and relatively stable development slow process;the excitative process of the sudden calamitous weather is the non conservative process of wet potential vorticity resulted from the dynamic non-equilibrium of various factors affecting potential vorticity in the condition of wet potential vorticity tending to zero, and it is the excitation process of the geostrophic departure in middle latitudes;the happening time of the sudden calamitous weather is often corresponding to the time of the turning change of one of the factors affecting potential vorticity, while the happening area is the area where the wet potential vorticity tends to zero.