Based on the weather background, atmospheric circulation evolution and the meteo rological conditions, including dynamic, thermodynamic, water vapor and the inst ability condition, the torrential rain in the southeast of Hunan Province caused by “BILIS” (the No.4 strong tropical storm in 2006) is examined by using the conventional observations and pentadaveraged meteorological elements. Results show that the ascending motion generated by the surface mesoscale convergence line and the dynamic lifting of topography strengthened the dynamic force of tor rential rain. In addition, the high temperature and high humidity provided ext remely good thermal energy, which is propitious to the development and the gener ation of convective instability ener gy. The eruption of the South China Sea m onsoon and the formation of monsoon trough in the southwest offered water vapor co ntinuously. Moreover, the continental high and subtropical high confronted each other and slow down the westward drift of “BILIS”. Besides the meteorological fact ors, the frangible geology environmental condition is the internal cause of the geological disaster induced by “BILIS”.