Based on the daily precipitation data of 66 meteorological stations over the middle and lower reaches of the Yangtze River, the Empirical Orthogonal Function (EOF) decomposition was used to analyze the extreme precipitation index of the moderate or above rain days and its formation reasons. Results show that the moderate or above rainy days over middle and lower reaches of the Yangtze River can be classified into the same anomaly pattern in the whole area and the reverse anomaly pattern in the south and north, but the both present interannual and interdecadal variation of the quasi-biennial period. The moderate or above days began to rise significantly in 1990s, and since 2000s, the north area of the Yangtze River exceeded the south; during 2000-2011, there still was "anticyclonic-cyclonic" anomaly of water vapor transportation in the meridional direction in the East China, and the anticyclonic water vapor transportation in Mongolia plateau was strengthened enough to cause the rain belt to stagnate in the north of the Yangtze River, and made the moderate or above rainy days in the south of the Yangtze River relatively less.