As a core component of the “Asian Water Tower” the Yangtze River headwaters region plays a strategic role in water security for the entire Yangtze River Basin. Under global warming, the hydroclimatic regime of the Qinghai-Tibet Plateau is undergoing profound restructuring, with glacier retreat, permafrost degradation, and lake expansion altering the storage forms and recharge pathways of water resources. Quantifying the climate-runoff coupling mechanism and projecting future trends are therefore urgently needed. Based on long-term observational data (1956-2022) from the Tuotuohe, Wudaoliang, Qumalai, and Zhimenda hydrological stations, this study systematically analyzes the spatiotemporal evolution characteristics of air temperature, precipitation, and runoff depth. A nonlinear regression model characterizing the synergistic response of runoff depth to temperature and precipitation was developed, and multi-scenario climate boundary conditions were designed to project future runoff changes. The results indicate that: (1) the warming rate in the source region reached 0.3℃/10 a, significantly exceeding the global average, with a clear vertical differentiation where high-elevation areas warmed faster than low-elevation valleys; (2) precipitation showed an overall fluctuating increasing trend (increase of 29.7-145.5 mm), yet exhibited pronounced inter-decadal variability and elevation dependency—low-elevation valley areas (Zhimenda) contributed the dominant share of water volume increase, whereas high-elevation hinterland stations (Tuotuohe and Wudaoliang) contributed the primary trend magnitude; (3) runoff depth increased with precipitation but was nonlinearly suppressed by rising temperatures through enhanced evapotranspiration and accelerated surface degradation, with the model achieving R² = 0.63 (P < 0.01); and (4) under future scenarios featuring precipitation changes of ±10% and temperature rises of 0-3.0℃, the runoff depth at the mountain outlet (Zhimenda station) is projected to fluctuate within approximately ±15%, suggesting relative overall stability. However, a critical threshold near a 2.0℃ warming marks the transition from a “meltwater gain” phase to a “glacier depletion” phase, posing a threat to long-term water resource stability. This study advances the understanding of hydrothermal coupling mechanisms in alpine basins and provides a scientific basis for climate-adaptive water resource management in the upper Yangtze River.