Abstract: A detailed analysis is made with a three-dimensional numerical simulation for the evolution of a heavy rain in a moist atmosphere on 12-13 June,1984.It is found that the increase of wind vertical shear may stir up a kind of internal gravity waves with approximate speeds of 23.1 m/s and wavelength of 500-600 km.The waves are a mechanism of triggering the heavy rain in the warm,moist atmosphere.