Abstract: Using a mesoscale model a numer ical study on a heavy rainfall case occurring in the Yang tze-Huaihe River Valley is made in this paper.The influence of the intensity of northeast erly wind in front of the Tibet an High at upper level on the low level wind field and development of mesoscale systems as well as heavy rainfall is invest igated.The model well repro duced the heavy rainf all process and the weather systems associated.And it indicates that the strong northeas terly flow around the High atupper troposphere will bring about notonly the streng thening of low level southeast erly wind,but also the appear ance of shear-line and mesoscale vortex at low level.The coupling of northerly wind at upper level and southerly wind at lower level constructs an indirect vertical circulation which is most favourable for the development of conv ective motions.Its ascending branch in the shear-line area is very strong and shows a pro nounced mesoscale characteristic.