Abstract: Analyzing the evolvement of divergent field is an approach to research the occurrence of torrential rains. However, the traditional divergence equation is obtained from the horizontal dynamic equations．In this paper， we use a new type of divergence equation and the NCEP/NCAR 1°×1° reanalysis data to analyze the barotropic nonequilibrium force and the baroclinic coupling forces at each stage of the persistent heavy rainfall event in Guizhou during 25-26 July 2007 and compare with the observed 1 hour and 6 hour accumulative precipitation and satellite data．The results show that this severe rainfall mainly happened under a nearneutral or weak unstable stratification．The increment of divergence caused by the barotropic nonequilibrium force should contribute to the occurrence of severe rain, which is the excited mechanism for this rain. In the course of the severe rainfall maintenance and enhancement, the baroclinic coupling forces between the dynamic and thermodynamic fields play a main role．The baroclinic coupling forces are the important factors for the heavy rainfall enhancement．The change of the divergence is consistent with the area,the strength and the position of the torrential rain happened 6 hours latter．As a result，the barotropic nonequilibrium force and the baroclinic coupling forces between dynamic and thermodynamic may serve as the indicators for the prediction of the severe rain，that may be used as a mathematical tool to diagnose the evolvement of divergent field． 