Summer monsoon index defined by land-sea presure contrast is compared to that by south-Asian monsoon circulation in characterizing the interannual variation of floods/droughts over the mid-lower valleys of the Yangtze. In terms of principal tropical circulation of east Asian monsoon defined is the summer monsoon index that is found to reflect well the interannual variation in the flood/droughts. The study shows that the difference in both higher and low index and the interannual variation has higher ability to identify the pattern of atmospheric circulation related to east Asian summer rainfall through the use of the index compositing and SVD analysis of northern 500 hPa height and synchronous SSTA at tropics in the context of 1982～1994 mean precipitation (precipitable water) and the 500 hPa height anomalies.