[Objective] Base flow is an important component of runoff. Studies investigating changes in river base flow and the associated mechanisms in the source region of the Yangtze River remain limited. The recharge conditions and influencing factors of river base flow have not been fully elucidated, and no relevant studies have yet examined the effects of climate oscillations on river base flow. Using observed hydrological data from the source region of the Yangtze River, this study aims to analyze the evolution of river base flow and its response to the Pacific Decadal Oscillation (PDO), explore the relationship between river base flow and PDO and the possible underlying mechanisms, and provide a reference for understanding the hydrological cycle and protecting water resources in the region under climate change. [Methods] The modified Kalinin method was employed to separate base flow from the measured runoff at the Zhimenda hydrological station. Correlation analysis and cross-wavelet analysis were used to analyze the response of river base flow to PDO and the correlation between them. Meteorological observations from the source region of the Yangtze River were analyzed to identify the main factors driving changes in river base flow. The potential mechanism by which PDO affected river base flow in the source region of the Yangtze River was explored from the perspective of atmospheric circulation. [Results] (1) Seasonal and annual river base flow in the source region of the Yangtze River increased significantly. Before 2000, base flow showed alternating high- and low-flow conditions but remained relatively stable. After 2000, river base flow showed a marked increasing trend that has continued to the present.(2) River base flow in the source region of the Yangtze River was negatively correlated with PDO. Annual base flow was significantly negatively correlated with the PDO index for the original series and the 3- and 5-year moving averages, with all correlations significant at the 99% confidence level. The two series exhibited an antiphase resonance at the 8-16-month timescale throughout the study period.(3) PDO had a significant influence on river base flow in the source region of the Yangtze River, with a typical negative correlation between them. When PDO was in a positive phase, a negative base-flow anomaly was more likely, indicating below-normal flow conditions. Conversely, when PDO was in a negative phase, a positive base-flow anomaly was more likely, indicating above-normal flow conditions.(4) Precipitation and temperature were the primary factors driving changes in base flow in the source region. Beginning around 2000, temperature increased significantly in the source region, followed by an abrupt increase in precipitation around 2008. The base-flow trend was generally consistent with the corresponding trends in precipitation and temperature.(5) Through teleconnections and atmospheric wave trains, PDO induced the formation of high-pressure ridges and anticyclonic circulation anomalies over the Tibetan Plateau, which in turn altered temperature, precipitation, and other meteorological variables in the source region. During the negative PDO phase, sea surface temperature anomalies in the central and western North Pacific induced anticyclonic circulation anomalies. Atmospheric teleconnections and wave-train propagation through the westerlies strengthened the anticyclonic anomalies over the Tibetan Plateau, resulting in surface warming, increased precipitation, and consequently increased river base flow in the source region of the Yangtze River. The opposite occurred during the positive PDO phase. [Conclusion] River base flow plays an important role in maintaining both aquatic and terrestrial habitats in the source region of the Yangtze River. This study provides an exploratory statistical assessment of the influence of PDO on river base flow. Future studies may combine numerical simulations, such as coupled climate-hydrological models, to further verify the chain of processes involving high-pressure ridges, anticyclonic circulation anomalies, changes in temperature and precipitation, and the river base-flow response.