Abstract: Quantitative identification of influencing factors and their contributions to the spatio-temporal variations of water balance is a prerequisite for the rational utilization of water resources.To investigate the water balance dynamic and its key influencing factors in the alpine ecologically fragile area, we take the period from 2001 to 2020 with significant hydrological changes in the Batang River Basin located in the midst of the Qinghai-Tibet Plateau as the research period.By comprehensively utilizing multi-source remote sensing data and observation data including meteorological stations and hydrological stations, the water cycle components including precipitation, evapotranspiration, and runoff in the basin are analyzed.The results showed that: ① precipitation and runoff have displayed significant increases in the past 20 years, with annual growth rates of 10.94 mm/a and 0.111×10⁸ m³/a, respectively.The evapotranspiration showed an insignificant increasing trend, with an annual increasing rate of 8.27 mm/a.② From the perspective of water balance, the difference between precipitation and evapotranspiration increased at a rate of 0.32 mm/a during 2001 to 2020, which was consistent with the increasing trend in surface runoff, indicating an overall increase in available water in this watershed.The average difference between precipitation and evapotranspiration of the entire watershed was 178.02 mm, with higher values in the southeast and lower values in the northwest.The differences between precipitation and evapotranspiration of various land use types were positive, ranging from 0.009×10⁸ to 6.767×10⁸ m³.③ Among abiotic factors, there was a significant positive correlation between ET and temperature (r=0.413).Among biological factors, ET was highly correlated with vegetation coverage.ET was mainly affected by meteorological factors in the non-growing season.And, ET was highly correlated with vegetation cover in growing seasons.The impacts of precipitation changes and human activities on natural runoff were 79.55% and 20.45%, respectively, indicating that runoff was relatively affected by climate factors.The results can provide a reference for scientific management of water resources and water balance regulation in alpine ecologically fragile area.