Based on the study of the satellite image, the radar echo and the surface meso-scale systems, the article mainly focuses on explaining how the "91. 7" Heavy Rain was developed and concludes that the rain-storm was a consequence of the interaction between the middle latitude westerly trough and the low latitude typhoon cloud-cluster;it also points out that if surface meso-scale convergence line (convergence center) appears in front of the surface meso-scale high energy tongue, then severe convective weather such as tornado,squall line,etc. would easily emerge.