With the aid of band pass filter,and scale separating method,the various mesoscale systems of a torrential rain occurred in Yunnan area from 30 May to 2 June in 2001 are analyzed.The results show that some mesoscale weather characteristics such as the instability energy,the streamline field and the dynamical structure before the torrential rain are very clear.It is highly importance for us to improve the forecast of torrential rain events.