Abstract: Revealing the interdecadal variation patterns of rainfall and the spatiotemporal evolution characteristics of rainstorms in the key region from the lower reaches of the Jinsha River to the upper section of the Three Gorges Reservoir is of great significance for water resource management in the upper Changjiang River and the optimal operation of the Three Gorges Reservoir. Using regression analysis, Mann-Kendall tests, power spectrum analysis, and composite analysis, we quantitatively analyze rainfall evolution patterns for the annual period, main flood season, reservoir impoundment period, and key drawdown period within three major sub-regions: the Yalong River Basin, the lower Jinsha River, and the mainstream of the upper Changjiang River. The results indicated that the Yalong River Basin exhibited a significant increasing trend in annual and drawdown-period precipitation, while the lower Jinsha River and mainstream of the upper Changjiang River showed decreasing trends across all periods. Periodicity analysis revealed a 5-year cycle during the flood season and a 13-year cycle during the impoundment period in the Yalong River Basin. A 3-year cycle was identified during the impoundment period in the lower Jinsha River, and 3-year cycles were found during the flood and drawdown periods in the mainstream of the upper Changjiang River. Composite analysis of rainstorms revealed such a typical pattern: precipitation typically initiates as moderate rain in the northern Jialing and Minjiang basins on the first day, intensifies into heavy rain while shifting southeastward to the mainstream, then continues eastward on the following day, weakening to moderate rain in the Three Gorges area before finally exiting the region. These findings provide crucial support for coordinated flood control and drought relief operations of cascade reservoirs and for adaptation to climate change.