Changes in glacier meltwater runoff in the Qilian Mountains situated on the northeastern edge of the Tibetan Plateau are important for sustaining water resources in the arid regions of Northwest China. Combining multi-period remote sensing, historical data and modeling, we evaluate the glacier mass and area changes as well as associated runoff change of all glaciers in the area. Glacier area shrunk by 516.8 km2 (similar to 26%, 0.53% +/- 0.15% a-1) between 1970 and 2020 and glacier surface elevation change was -0.35 +/- 0.04 m a-1 between 2000 and 2014. In general, glaciers in the eastern Qilian Mountains retreated faster than those in the western part. A mass balance model calibrated against geodetic estimates derived from DEM differencing was used to reconstruct annual glacier mass changes and runoff from 1990 to 2017. Across the 11 glacierized large-scale river basins (0.02%-2.5% glacierization), the index-based glacier runoff ratio relative to basin water input ranged from 0.1% to 6.2%. Results show an increase in glacier runoff volumes during 1990-2001, but no statistically significant trend thereafter (2002-2017) despite increasing glacier thinning. This slowdown is attributed to decreasing glacier area as the glaciers retreated. Overall, our findings highlight that while glacier runoff continues to play a critical role in sustaining river runoff in the Qilian Mountains, its buffering effect is weakening as glacier runoff approach peak water, implying increasing vulnerability of regional water resources to precipitation variability under future climate change.