A variety of data are used to analysis and diagnose torrential rain during August 18 and 19, 2010, which brought secondary disasters such as debris flows. By using the nonhydrostatic WRF (weather research forecast) model, this torrential rain is simulated in high resolution, and the simulation data are used to make a preliminary diagnosis of this rainfall. The results show that: (1) This torrential rain event was characterized by its short duration, heavy intensity in short-term precipitation, and localization, all of which show mesoscale features spatially and temporally. Heavy rain closely associated with the terrain mainly occurs in the vicinity of the Wenchuan-Yaan fault zone steep terrain.(2)There exist two ridges in a trough atmospheric circulation pattern in the middle and high latitudes. The same phase super-position of the northwest subtropical high and an anticyclonic ridge form typical weather patterns with high pressure to the east and low pressure to the west. These systems, spreading eastward slowly, provide a favorable large scale circulation situation for precipitation processes. The development of a southwest vortex and its interaction with the strong wind belt of the subtropical high western edge strengthen the upward-effect of the western Sichuan steep terrain and contribute to the occurrence of heavy rain. The occurrence, development, merger, and strengthening of several mesoscale cloud clusters in the Sichuan basin is directly related to the occurrence of heavy rain. (3) The WRF model successfully simulates the precipitation process, and our analysis of the simulation results indicates that, under the influence of the development and evolution of the large scale circulation background, the movement of a stable stratification cover, the development or decrease of lower southeast wind, and the accumulation and release of unstable energy dominate the formation and cessation of precipitation.