Mesoscale filter analysis for one flash flood process in Weihe river valley is performed with the route sounding data and NCEP 1°×1°reanalysis data, based on the 25-point lowpass filtering technique, so that a model for flash flood of Weihe river valley was set up by using of meso-scale circulation evolution characteristics. The results show that vortexes with scale less than medium were hidden in the average southwest stream. Its movement direction and development level could determine the area and the intensity of heavy precipitation. The coupling of westerly fluctuation at high level with south fluctuation at low level resulted in a mesoscale secondary circulation loop, its rising branch provided sufficient impetus for the heavy rainfall. And the reasonable allocation of the low level jet, mesoscale circulation at 700 hPa and west fluctuation at high level was the necessery factor for the mesoscale secondary circulation loop. The surface mesoscale convergence line was the trigger mechanism for this flash flood.