By means of high-resolution MRI TBB data and NCEP reanalyzed data,an analysis is made of 13 frontal large-scale rainstorm processes influenced by the summer monsoon system in flood season(May to July) from 1980 to 2002 in Guangxi Autonomous Region.Through composite analysis combining with case studies,the evolutionary characteristics of the influencing systems(upper-level jet, low-level jet,South Asia High,etc.) and the dynamical and vapor characteristics of rainstorms are analyzed by using the high-resolution TBB data.The results indicate that Guangxi locates,during rainstorms,in the positive divergent area within the right exit region of the high-level jet,the intensive updraft region at 500 hPa,to the left side of the low-level jet;the coupling between highlevel and low-level jets is the trigger mechanism of rainstorms;further more,the analysis of wet potential vorticity indicates that heavy rainstorms are more likely to occur while the vertical component of moist potential vorticity being less than 0 and the vertical component of moist potential vorticity at the isobaric surface being greater than 0 near 700 hPa.