By using dense convective observational data, satellite images, radar data and NCEP/NCAR (1°×1°) reanalysis data, the weather systems for the three extreme heavy rainfalls are diagnosed and analyzed. The results show that westerly trough at 500 hPa and low level warm shear line are the main impact system for the three heavy rainfalls. Before the rainfalls, high temperature, high humidity and convection instability exist in lower atmosphere. The ascending movement caused by convergence of low level shear line and the warm and wet advection and the surface wind convergence line trigger the release of convection instability energy, producing strong convection and extreme rainfall. The strong vertical wind shear makes convection systematically develop. During the heavy rainfalls dry and cold airs invade from middle upper level so that convection instability gets intensified. On the other hand, the dry and cold airs with high potential vortex intruding from middle upper level induce mesoscale vortex to develop in low level atmosphere so that convergence and ascending movement increase. That the warm and wet airs in low level helically converge and encounter the dry and cold airs from middle higher level, so the water vapour condensation and rain intensity increase. The periods when the dry and cold airs intrude from middle higher level correspond with the periods of extremely strong rains. Terrain plays an important part in local and short time extreme heavy rainfalls. The position and intensity for 500 hPa westerly trough and 850 hPa warm shear line are different and the intensity and tracks of the dry and cold airs at upper level are different, the convective cloud’s development, evolution, movement and internal structure are different, causing the difference of rainstorm areas and intensity.