The diagnose and calculation of synoptic large-scale condensation function precipitation and moisture flux divergence are conducted on the isentropic coordinates system with the re-analysis data of T213 L31 model atmosphere.Meanwhile,the ideal design and diagnostic analysis are made of the covection-unstable precipitation for the entire layer and the wet instability for air parcels(mass).The calculation and analysis of the Meiyu-front heavy rainfall over the Changjiang-Huaihe region in July 2003 indicates that the rainfall amounts leading from both types of synoptic scale precipitation movements(condensation function and moisture flux divergence) are not large enough to form heavy rain;the moist air-layer convective precipitation movement may induce heavy rain but not rain gust;only the air-mass wet-instability precipitation movement is strong enough to cause the self-organized and positive-feedback rain gust.It is made known that although there are above-mentioned two large-scale precipitation movements on the Meiyu front in the Changjiang-Huaihe basin in July 2003,the highly moist air masses(precipitable water) maintain long time and transport rapidly,leading to the occurrence of non-isentropic saturation-adiabatic motions and the convection-unstable precipitation of the moist air layer and the air-mass instability precipitation,which are the synoptic causes of the heavy rain event.