Abstract: With the rainfall data in Guangdong,the grades of drought and flood in both first stage (Apr.～June) and second stage (Jul.～Sep.) of rain season in Guangdong have been partitioned with Z-index. The grade series in the first stage have 24 years,6～8 years and 3～4 years cycle. The grade series in the second stage have a 10-year cycle,which have been found by Morlet wavelet transform. With compariton between drier and wetter years in the first stage,it has been found that the major reason of rainfall more than normal is the interaction between the stronger southerly in South China Sea and deeper East Asia trough and stronger high jet in East Asia. The major reason for less-than-normal rainfall is that cold air is weaker. The SST in warm pool of western Pacific has an important impact on that interaction. The warmer the SST,the less the rainfall will be in Guangdong. In the paper,we also discuss the forecast technology. It suggests that SST in the warm pool may play as a significant signal for forecasting the drought and flood in the first stage. The atmospheric circulation anomalies in the middle and higher latitude of both Hemispheres shall be taken more notice for forecasting the drought and flood. With compariton between drier and wetter years in the second stage,it has been found that the stronger (weaker) Asia summer monsoon system is a most important factor for the wetter (drier) and the number of landing typhoon is also another influencing factor. It suggests that the height anomalies of preceding May at 500hPa in (40～50°N,160～140°W) may play as an significant signal for forecasting the drought and flood in the second stage. The atmospheric circulation anomalies in the middle and higher latitude of Southern Hemisphere can affect the drought and flood through their impact on the Indian summer monsoon.