Abstract: The vertical distribution of the humidity field in the torrential rain process is analyzed using the sounding data of three weather stations in Hunan Province during the period of June, 1974~1987. It is found that there was a layer in the middle of troposphere (600~400 hPa) before beginning of heavy rainfall. Furthermore, the feature, evolution and cause of dry layer, and their mechanism which led to heavy rainfall, are investigated. The results show that the humidity change in vertical direction was very quickly, the dry layer was formed before the heavy rainfall and would distort gradually after the heavy rainfall. Additionally, the dry layer formed in heavy rainfall was clearly different from that of the other severe convection weathers (such as severe storm and hail). There was an instable convection layer below the dry layer and a convective stability layer over the dry layer.