This paper studied the natural law of the drought and flood cycle by comparatively analyzing the hydrological characteristics of interannual rainfall series at the Wuxi Station before and after the climate transition period. The results show that the interannual rainfall process of the Wuxi Station indicates a mutation before and after 1978， which confirms the existence of the climate transition period. The interannual drought and flood periodicity of the Wuxi Station is a part of the natural evolution law of hydrology， of which the consistency of the time series is not changing due to the influence of human activity. The proposed segmentation method breaks the traditional thinking of drought and flood irregular. This study solved the interannual periodicity of drought analysis problem of the Wuxi Station， realized simulation forecast of the drought and flood trend which is a help for mid- and long-term hydrological forecasting and climate prediction in the Taihu Lake area.