This paper analyzes the interdecadal variation of water vapor transport over East Asia and its impacts on rainfall over eastern China in summer by using EOF analysis method, NCEP/NCAR monthly reanalysis data and monthly rainfall data covering 160 stations over China from 1951 to 2004 in summer. The results show that the main distribution mode of summer rainfall over eastern China is south and north flooding/middle drought from the mid- and late of the 1970s to the beginning of the 1990s and presents a south flooding/north drought pattern since the beginning of the 1990s. The interdecadal variation of summer rainfall over eastern China is very significant and rain belt shifts southward obviously since the 1990s. The leading mode of EOF and its time series coefficients reveal that the meridional water vapor transport over east of East Asia Mainland is characterized by a turning point from strong to weak and a decadal weakening trend since 1974. The zonal water vapor transport over India Mainland and northern part of Bay of Bengal, middle and lower reaches of Yangtze River Valley, South China Sea and tropical Pacific region is characterized by a turning point from weak to strong and a decadal enforcing trend since 1973. The temporal-spatial evolution in summer water vapor transport over East Asia is one of key factors that trigger a shift from south and north flooding/middle drought to south flooding/north drought, which leads to the induction of the rain belt shifting southward.