Abstract: Based on the runoff data from Tuotuo River hydrological station, the precipitation and temperature data from the Tuotuo River meteorological station, the ERA-interim reanalysis data in summer from 1979 to 2016, the influence of the position of West Asian Subtropical Westerly Jet (WASWJ) in summer on the runoff over Tuotuo River Basin in the source region of Yangtze River and the corresponding physical mechanism have been investigated, using the methods of correlation, regression, and EOF analysis. Results show that the latitudinal position of the WASWJ has a significant influence on runoff of theTuotuo River Basin in summer. In detail, the southward (northward) WASWJ axis could result in more (less) runoff over there by modulating the change of precipitation. When the jet position is further south, the South Asian high inclines to more eastward position, and the anomalous negative geopotential height and significant ascending movement over the Tuotuo River Basin is conductive to the development of the plateau low-value system, resulting in more precipitation. Meanwhile, the anomalous southwesterly flow is favorable for the water vapor transport from the lower latitudes into the study region, leading to increased precipitation and more runoff formation, and vice versa. Further analysis shows that the Arctic Oscillation (AO) in summer can significantly affect the latitudinal position of WASWJ by affecting the temperature variations in the upper and middle troposphere from West Asia to Central Asia, and there is significant negative correlation between them. The results of this research have important implications for the water resources management in the source region of Yangtze River under global warming.