Abstract: It is pointed out in the paper that the geographical distribution of source/sink for water vapor can be correctly reflected only by seasonal mean water moisture flux divergence summed up vertically.Results show that the west Pacific(120-133°E) and the Bay of Bengal are the sources of water vapor in Asian monsoon area in August,in which,the five strongest ones are located in east of Philippines,Sulu sea,Huang Hai Sea,Andaman Sea and Brahmaputra Valley.The South China Sea is a sink,not a source of water moisture.The rainfall in North China is mainly under the influence of two Monsoon systems in Asia and the water source in Huang-Hai Sea and Brahmaputra Valley.