The ETA model of the Chengdu Regional Meteorological Center has its function of forecasting the heavy rainfall occurring in the Northwestern Sichuan Basin on 18 Sept. of 2001. The model can reveal the movement and stability of the rainfall area ,and the tendency of the alleviation in rainfall intensity except a little weakness in the intensity and difference in the rainfall area. The numerical experiment shows that during the early stage of the process, southwest and southeast air flow in the middle and lower troposphere are very important to the rainfall while this is not true for the cold air from north.