Mechanism is compared for two rainstorm processes in different places of the Yangtze River valley in Jul. 21,1998. The torrential rain in Sinan area of Guizhou Province in the upper valleys of the Yangtze River is under the influence of southwest vortex,low-level westerly jet and south Asia high. Rainbelt locates on the left-mid-hand side of the low level jet and the main vapour source is from the Bay of Bengal. But the torrential rain in Wuhan in the mid valleys of the Yangtze River is principally resulted from the coupling of boundary-layer southerly jet,low-level westerly jet and high level westerly jet. Its major vapour source is from South China Sea. Non-thermal wind convective instability at low level and non-thermal wind symmetric instability at high level are the mechanism of torrential rain.