Using 1951～1991 500 hPa height and SST, and 1980～1991 T_BB data, study is undertaken of the features of east Asian monsoon seasonal transition in dry and wet years by means of composite analysis, indicating that different SST patterns and difference in the related air-sea interaction are responsible for slow northerly march of western Pacific subtropical highs on a seasonal basis, with the stronger ridge steadily located around 25°N, causing the summer monsoon rainband persisting in the Jiang Huai drainages for flood. Further, addressed are the effects of the Indo China convection and the change in a equatorial high pressure band as forerunners upon the Jiang-Huai flood/drought.