Abstract: This study further clarified the influence of the westerlies and the South Asia monsoon on both the water vapor transport and precipitation in the Three-River Headwaters Region. ERA-20C reanalysis datasets were used to perform an empirical orthogonal function (EOF) decomposition of the water vapor transport flux of the Three-River Headwaters Region during the rainy seasons of 1948-2010. The EOF mode and index were acquired that can indicate whether the water vapor transport in the investigated region is controlled by the westerlies or the South Asia monsoon. The spatial patterns of water vapor transport and precipitation in the region were found under control of the westerlies or the South Asia monsoon. Under control of the westerlies,the water vapor transport direction inside the region was NW-SE. The westerlies and the South Asia monsoon converged at the south boundary of the region;furthermore,precipitation increased significantly in the east and south of the region (i.e.,the majority of the Lancang River headwater and east of the Yellow River headwater). Under control of the South Asia monsoon,the water vapor transport direction was S-N,the two water vapor transport paths converged in the north,and precipitation increased significantly in the north (i.e.,the majority of the Yangtze River headwater,and the north of the Yellow River headwater). The westerlies and the South Asia monsoon both exerted a vital influence on precipitation in the Three-River Headwaters Region. Water vapor transport paths controlled by either the westerlies or the South Asia monsoon led to positive precipitation anomalies in different parts of the region.