Abstract: The characteristics of drought and floods grades, long-term change trend and affecting factors of the first raining period (April to June) and the second raining period (July to September) in Guangzhou are analyzed by using the monthly rainfall data in Guangzhou for the years of 1908-2000, Hadisst SST datasets and NCEP reanalysis data. The results show that continuity and intermittence of the drought and flood events of the first raining period were more obvious than that of the second one in Guangzhou over the past 100 years. Precipitation of the first and the second raining periods underwent four wet and three dry periods and two wet and two dry periods, respectively. The annual precipitation presented an ascending trend in the first raining season but a descendent one in the second raining season. Because they did not exceed the 10% significance level, so they still locate in the range of the natural oscillation. The mechanism analysis indicated that air-sea interactions make it possible for preceding spring (April to June) SST anomalies in the Western Pacific warm pool to affect the coming general circulations (July to September), causing precipitation anomalies in the first raining season in Guangzhou. The teleconnection between the South China Sea summer monsoon and the 500hPa geopotential height field affects the precipitation anomalies in the second raining season in Guangzhou.