Abstract:Guangdong suffered the heaviest rainfall during late May to mid June 2008 (so c alled “dragon boat” precipitation), according to records since 1951. Result o f diagnostic analysis shows that in this case the circulation situations of the middle and high levels in Asia and Europe are two trough and one ridge circulation pattern and the western Pacific subtropical high is weaker t han normal and expands eastward. Four main heavy rain periods in this case are a ssociated with the influence of westerly trough and southward movement of low level shear line . The variance of low level jet has a close relationship with the occurrence of the rainstorm. The cold air moving down from the mid latitude areas and joinin g up against moist southerly from the Bay of Bengal and the South China Sea prod uces the vapor convergence at southern China coastal areas and the anomalous loc al meridional circulation, which causes the continuous heavy rainfall. Three of four heavy rainfall periods are associated with the instable stratification.