The vapor sources of the heavy rain during August 1996 in terms of real data was studied.The result points out that the vapor of the torrential rain comes from the South China Sea and the Bay of Bengal as well as typhoon-low itself carrying moisture.Finally,the heavy rain sensitivity to vapor by virtue of MM4 model was analysed.It is suggested that a little variety of vapors can bring large change of rainfall and latent heat releasing has positive feedback effects to heavy rain.