Abstract: Using NCEP/NCAR data and 40 years precipitation data of QinhaiXizang Plateau meteorological stations. The results showed that the main water resource of the origin area of three rivers (Changjiang, Huanghe, Lancangjiang) in Plateau is the southwest wet airflow by East Asia and Indian monsoon and the secondly is the west airflow from Middle Asia high pressure of west boundary and the third one is the north airflow in the westerlies. These three different airflows of large-scale circulation background influx to the three rivers in Plateau and make thisregion lie in the convergence area of vapor from June to September. At the same time the ground layer of the source of three rivers in Plateau maintain stationary weather systems such as shear and vortex and so on under the dynamic effect of large topography of Qinhai-Xizang Plateau and form the source of rivers in this area being perpetual rainfall. The vapor flux of main importing boundary of the source of three rivers in Plateau present descending trend in near 40 years and will influence on the precipitation change of the origin area of three rivers in the future.