Through 3D nesting of a limited-area fine-mesh model and a T₄₂ L₉ spectral model,numerical study is undertaken of Meiyu-season torrential rainfall over the Changjiang-Huaihe rivercatchment from 0800 BST,10 to 0800,11 July,1991,with the influence of the low-latitude circulation systems and the related vapor passage on the Meiyu precipitation investigated.Results showthat the Bay of Bengal water source and the SW monsoon-formed passage contribute more greatlythan do the source of the South-China Sea and the SE wind-caused passage;both the sources,if-coexistent,produce nonlinear superimpositional effects,causing the enhancement of the rainfall;the change in the vigor of the typhoon and monsoon depression circulations is of importance tothe formation of the torrential rainfall as well.