Abstract: The runoff variation in the upper reaches of the Jinsha River is crucial to water resource management in the Changjiang River Basin.Its long-term changes are influenced by multiple climatic factors; therefore it is necessary to analyze its driving factors and mechanisms.Based on station data, global sea surface temperature (SST) data, and reanalysis data, this study analyzed spatiotemporal characteristics of runoff in the upper Jinsha River, and revealed the driving mechanisms of Pacific and Atlantic SST anomalies on runoff during the key snowmelt period (April to June) and the main flood season (July to September).The results indicate that: ① The annual mean runoff in the upper Jinsha River shows a significant fluctuating upward trend. ② The primary controlling factors of runoff recharge exhibit significant monthly differences.From April to June, the contribution of seasonal snowmelt to runoff increases significantly, while precipitation becomes the dominant factor controlling runoff variation at the Shigu Station from July to September. ③ The effects of SST anomalies on runoff vary by region and season.In March, cold SST anomalies in the equatorial Pacific trigger an anomalous low-pressure system and warm advection over the upper Jinsha River, which significantly enhances precipitation and snowmelt during the snowmelt season, thereby increasing runoff.In June, the North Atlantic Tripole (NAT) SST anomalies modulates the Eurasian teleconnection wave train, thereby intensifying southwesterly moisture transport and promoting increased precipitation during the main flood season, ultimately leading to higher runoff.These findings provide theoretical support for seasonal runoff prediction in the upper Jinsha River.