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The observed data of precipitation at 160 observational stations of China,the ERA-40 reanalysis data and the Empirical Orthogonal Function(EOF) and the entropy spectral analysis methods are applied to analyze the interannual variations of summer(JuneAugust) rainfall in China and water vapor transport fluxes over East Asia.The results show that there is an obvious oscillation with a period of two or three years,i.e.,the quasi-biennial oscillation,in the interannual variations of summer monsoon rainfall in China,especially in the eastern and southern parts of China including South China,the Yangtze River valley and the Huaihe River valley and North China.And it is also shown that this oscillation is closely associated with the quasi-biennial oscillation in the interannual variations of the water vapor transport fluxes by summer monsoon flow over East Asia.Furthermore,the interannual variations of sea temperature in the surface and subsurface of the tropical western Pacific are analyzed by using the sea surface temperature(SST) data from the NCEP/NCAR reanalysis dataset and the sea temperature data in the subsurface of the western Pacific along 137° E from Japan Meteorological Agency,respectively.And it is revealed that there is also a significant quasi-biennial oscillation in the interannual variations of thermal state of the tropical western Pacific.In this paper,the correlative and composite analysis methods are applied to discuss the influence of the quasi-biennial oscillation of thermal state of the tropical western Pacific on summer rainfall in China and water vapor transport over East Asia,and it is shown that the quasi-biennial oscillation in the interannual variations of thermal state of the tropical western Pacific has a great impact on the East Asian summer monsoon and the water vapor transport driven by the monsoon flow.Besides,the influence of the quasi-biennial oscillation in the interannual variations of thermal state of the tropical western Pacific on the quasi-biennual oscillation in the interannual variations of the summer monsoon rainfall in China is simply discussed by using the teleconnection theory of the East Asia/Pacific(EAP) pattern. From the above-mentioned analyses,the physical mechanism of the quasi-biennial oscillation of summer rainfall in China may be summarized as follows: If the thermal state of the tropical western Pacific is in a warming state during a winter,then the convective activities will be intensified around the Philippines in the following spring and summer,which can cause weak summer monsoon rainfall in the Yangtze River and the Huaihe River valleys through the EAP pattern teleconnection.And due to the intensification of the convective activities around the Philippines,a strong convergence of atmospheric circulation will appear over the tropical western Pacific.This will trigger a strong upwelling in the tropical western Pacific.As a consequence,the thermal state of this region will turn into a cooling one in the following winter.On the other hand,since the tropical western Pacific will in a cooling state during the following winter,the convective activities will weaken around the Philippines in the spring and summer of the third year,which can cause strong summer monsoon rainfall in the Yangtze River and the Huaihe River valleys through the EAP pattern teleconnection.And due to the weakening of the convective activities around the Philippines,a divergence of atmospheric circulation will appear over the tropical western Pacific in the spring and summer of the third year.As a consequence,the thermal state of these ocean regions will again turn into a warming one in the winter of the third year.