Abstract: Cryospheric change on the Tibetan Plateau is reshaping its hydrological processes and threatening regional water security. The Lhasa River, which is the largest tributary of the Yarlung Tsangpo River, is a typical example of a river in a meltwater-fed basin that is highly sensitive to climate change. Using the VIC-glacier model constrained by multisource datasets, including meteorological and hydrological observations, glacier inventories, and satellite snow products, the runoff components of the Lhasa River Basin during 1975—2016 were reconstructed and partitioned. The daily discharge was consistently reproduced at four gauges (Nash-Sutcliffe efficiency: > 0.70; relative error: < 10%). At the downstream gauge on the Lhasa River, mean annual runoff was dominated by rainfall runoff (75.8%), with snowmelt and glacier melt water contributing 22.2% and 2.0%, respectively; at the upstream Yangbajing gauge, glacier melt water accounted for 41.8% of runoff. Over the study period, snowmelt runoff declined substantially (− 0.693 mm/a), whereas glacier meltwater runoff increased (0.047 mm/a). Precipitation was the primary control on runoff variability, while air temperature reshaped runoff composition by regulating meltwater processes, exhibiting pronounced seasonal contrasts. These findings provide a scientific basis for assessing water-resource changes and supporting adaptive management in high-altitude basins.