An air parcel method is used to analyse the symmetric instability mechanism and the circulation caused by it.The diagonoses of the case of torrential rain indicate that there is symetric instabilitiy before the precipitation and in the process of it in some area.So it could be concluded that the circulation induced by symmetric instability could induce and strenthen the torrential rain.A method to conform whether there is symmetric instability and the power of it by using conventional observation and radiosonde data is introduced.Furthermore,the forecast of precipitation and its intensity can be made by the method and the accuracy of torrential rain and flood forecast can be greatly improved.