Abstract: One of key scientific issues in Yangtze River flood forecasts is the persistent water vapor transport to maintain the rain belt and its mechanism. Therefore the moisture source and its transfer sensitive region are the clues to the flood forecast in China. Some studies show that the dynamic and thermal effects of the Tibetan Plateau play an important role to the Asian monsoon activities, and they are also the key factors for water vapor transport mechanism for the Meiyu rain belt in the Yangtze River valley. The region in the south of Plateau exist a distinct warm/wet airflow from the Indian Ocean, the South China Sea and the tropical West Pacific. Under the dynamic and thermal effects of the plateau, the southerly water vapor flow from the lower latitudes turns right to Yangtze River valley at the southeast of Tibetan Plateau during its northward transit. Which form the specific water cycle process in the plateau peripheral areas. This is the conception of water vapor transport “forwarding station”, put forward by Prof. Xu xiangde. In this paper, a “box” model is adopted to discuss the whole layer water vaporbudget feature at the edges of middle and lower reaches of Yangtze River valley. The results show that the main input part of water vapor transport in summer is from south and west edges with the main output part at the east edge. Of course, the composite water vapor budget of the region has close relationship with the regional rainfall. Moisture flux components are analyzed in the key areas south and east to the Plateau. It's found that the northward water vapor component in the south of the Plateau is correlated significantly to the eastward in the east of the Plateau. Furthermore, the influencing areas of these two kinds of moisture flux components are discussed. The results show that they can both affect on different degree the summer rainfall, as well as the composite water vapor budget of downstream regions. These results not only verify further the opinion of water vapor transport “forwarding station”, but also reveal that the moisture flux components transform effect possesses influence on the summer rainfall of Yangtze River valley. This is one of the key factors to understand the causes of regional flood and will provide some scientific proof for flood forecast of Yangtze River valley.