The regional wind index influencing September precipitation in western China has been defined using the 700 hPa u and v components of NCAR/NCEP reanalysis data of 1961 to 2006.There are three regional wind field indices:southwest,southeast,and north,and these indices reflect the change of East Asian monsoon.The relationship between the indices was studied,and results show that they not only have a close relationship,but also have independence.Moreover,there is an obvious relationship between the wind indices and the autumn in particular the September precipitation in western China.The effect of wind indices on rain occurrence is most different:the influenced area of the southwest wind index is larger than that of the southeast wind index,and the southwest wind index is a controlling factor on autumn precipitation in western China.The weakening of southwest wind is the main cause resulting in reduction of autumn precipitation on the east side of the Tibetan Plateau.