Abstract: In this paper,by using generalized normal mode method the symmetric instability criterion which can be used not only in the constant parameters but also in the variable parameters are derived from the energy equation on the symmetric disturbance. If the thermal wind is balance or quasi-balance,the criterion was consistent with the criterion which is obtained by Emanuel with parcel method and Hoskins with normal mode method. It is showed in here that in the generating stage of disturbance,the symmetric instabilitic development of disturbance is connected with that the energy of the basic field is transfered into the energy of the disturbance field.