Abstract: Based on East Asian summer monsoon (EASM) index defined by Huang Gang et al., the differences of atmospheric circulation, atmospheric heat source and external forcing (i. e., SST) between strong and weak monsoon years are discussed.It is found that SSTA in Pacific appears La Niña pattern in winter preceding a strong EASM until the summer. That is to say, the SST in the equatorial central and eastern Pacific is low, while that in the Western Pacific Warm Pool is high.The convective activities over the equatorial eastern Indian Ocean, Sumatra and Warm Pool become obviously stronger than weak monsoon years.Meanwhile the atmosphere over South Asia, extending from the Indian subcontinent, through the southern part of the Tibetan Plateau, the Indochina Peninsula to South China, is abnormally heated.Furthermore, the analysis of mean tropospheric temperature anomalies shows that the heat contrasts between sea and land intensify.It is advantageous for the occurrences of strong EASM.In addition, the ascendant flow near Warm Pool is strong because of the active convection.So the Walker circulation becomes intense. The Asian subtropical westerly jet and subtropical high are located northward while the strong EASM advances northward.North China is located at the south side of the Asian subtropical westerly jet during July and August.So the precipitation over North China is abundant.Another rain belt is located at South China.But the precipitation over the Yangtze and Huaihe River Basins is short.On the other hand, winter SSTA in Pacific shows El Niño pattern preceding a weak EASM.The SST in the equatorial central and eastern Pacific and tropical Indian Ocean is high whereas that in the Warm Pool stretching through northwestern Pacific is low.This SSTA pattern lasts until the summer.As a result, the convective activity near Warm Pool becomes weak.Moreover, from April to July the atmosphere over the equatorial central and eastern Pacific is abnormally heated.But the atmosphere over South Asia, extending from the Indian subcontinent to South China, is cold.At the same time, the land-sea thermal difference between Asian land and the ocean weakens.It means a weak EASM.The center of convection activity shifts eastward into the central Pacific.Accordingly the Walker circulation becomes subdued.The ascendant flow over Indochina Peninsula and Philippine is weak.The Asian subtropical westerly jet and subtropical high are located southward while the weak EASM advances northward.The Yangtze and Huaihe River Basins are located at the south side of the Asian subtropical westerly jet during July and August, the precipitation over this region is abundant.Also, the physical scenario of atmosphere and SST anomalies associated with the interannual variability of the EASM are given.