Based on precipitation and ERA5 reanalysis datasets from 1981 to 2020, this study analyzed the variation characteristics of precipitation at different time scales over the Three-River Headwaters region (TRHR) and the Yarlung Zangbo River basin (YZRB) and their responses to the Tibetan Plateau summer monsoon. Results are shown as follows: (1) The seasonal variation in precipitation over the TRHR and YZRB shows a bimodal distribution, and the peaks appear in early July and late August. The interdecadal transitions in summer precipitation occur in the early 21st century, especially the TRHR precipitation increases significantly during the recent 20 years. The onset time of summer monsoon in the Dynamic Plateau Monsoon Index (DPMI) and the Zhou Plateau Monsoon Index (ZPMI) is earlier than the precipitation increase period over the TRHR and YZRB. The interannual variation in summer precipitation over the TRHR correlates well with two plateau summer monsoon indices. Although the TRHR is close to the YZRB, the summer precipitation of the TRHR is considerably more affected by the Tibetan plateau monsoon than 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 and southeast winds over the TRHR so that the warm and humid air from the South can go deep into the hinterland of the plateau, resulting in stronger water vapor convergence. In wet YZRB years, there is no obvious anomaly in the pressure field near the YZRB or the Tibetan Plateau. The water vapor transport over YZRB mainly has two paths. One is the southwest path from the Bay of Bengal along the south slope of the plateau, and the other is the northwest path from Central Asia and through the plateau. The two paths converge on the east side of the plateau and continue to transport eastward.