Based on T213,NCEP/NCAR reanalyzed and conventional observation data, the general circulation and the physical field of the heavy precipitation on August 19 are analyzed at first. The result shows that this event is a typical southwest flow type caused by West Pacific subtropical high extending westward. The moisture mainly moved from the Bay of Bengal and the South China Sea. The moisture transport in low-and medium-level is very strong and the convergence occurs in Ningxia and east of Gansu Province. Second, with the three-way nested high resolution mesoscale NWP model MM5 to simulate this event by two methods, the four-dimensional data assimilation model is proved to be of quite good quality in circulation and precipitation forecasts.