Based on the observations for 1991 and the historic data for the years when severe drought-flood damages happened,a comparative analysis is made in this paper. The influences of the onset of the Indian south-west monsoon and the East Asian monsoon, the interplay between the upper-level and low-level monsoon flows, the changes in air temperature over the Qinghai-Xizang plateau and in sea surface temperature,and the anomalous variations of the general circulation and the East Asian monsoon circulation on the drought/flood in the Changjiang-Huaihe valley are discussed. Analysis indicates that the summer monsoon anomaly is an important factor causing the severe drought/flood in the Changjiang-Huaihe valley and its main features are the early onset of the strong southwest monsoon and the late prevalence of the southeast monsoon in the flooding years and the opposite situation in the drought years. It is also found that the characteristics of the confluence of the three monsoon flows,i, e., the position of their confluent point and the orientation of the confluent flows are related with the site of heavy rainfall.