Based on precipitation and ERA5 reanalysis datasets from 1981 to 2020, this paper analyzed the variation characteristics of precipitation at different time scales over the Three-River Headwaters region (TRHR) and Yarlung Zangbo River basin (YZRB) and their responses to the Tibetan Plateau monsoon. Results show that: (1) The seasonal variation of precipitation over the TRHR and YZRB both show bimodal distribution, the peaks appear in early July and late August. The interdecadal transitions of summer precipitation occur in the early 21st century, especially the TRHR precipitation increases significantly during recent 20 years. The onset time of summer monsoon in Dynamic Plateau Monsoon Index (DPMI) and Zhou Plateau Monsoon Index (ZPMI) are ahead of precipitation increase period over the TRHR and YZRB. The interannual variation of summer precipitation over the TRHR has a good correlation with two plateau summer monsoon indexes.. Although the TRHR is close to the YZRB, the summer precipitation of TRHR is much more affected by the Tibetan plateau monsoon than that of YZRB. When the Tibetan Plateau summer monsoon strengthens (weakens), the TRHR precipitation is more (less). (2) In wet TRHR years, the South Asian High is stronger and more eastward, while the pressure at low-level over the main body of the plateau is lower than in dry years. These situations are conducive to the intersection of southwest wind and southeast wind over the TRHR, so that the warm and humid air from the South can go deep into the hinterland of plateau, resulting in stronger water vapor convergence. In wet YZRB years, there is no obvious anomaly in pressure field near YZRB or the Tibetan Plateau. The water vapor transport over YZRB mainly has two paths, one is the southwest path which from the Bay of Bengal along the south slope of the plateau, and the other one is the northwest path which from Central Asia and through the plateau. The two paths converge at the east side of the plateau and continue to transport eastward.