By using the ETA model,a numerical simulation and experiment was conducted for a torrential rain in the northenwest of Sichuan basin on 19 August,1998.The results show that the ETA model is of better abilities on the presage of the system movement,the intensity and area of precipitation.During the torrential rain,low vortex over the plateau has the greater effect on the intensity and area of precipitation.Topograply has a close relation with the area of the torrential rain.The existence of small high pressure between Gashu and Shaanxi avails increment of the intensity of precipitation.