Oasis expansion-driven by both climate change and human activities-profoundly impacts ecosystem services in arid inland river basins. However, previous studies have predominantly focused on broad-scale climatic or land use changes, lacking a fine-scale spatiotemporal analysis of how oasis expansion specifically drives water-yield dynamics in such basins. To fill this critical gap, this study uses the Ebinur Lake Basin as a representative case, employing a locally calibrated InVEST model combined with Geodetector analysis to systematically quantify the spatiotemporal evolution of water yield and examine the influence of oasis expansion from 2000 to 2020. Water yield is a vital provisioning service underpinning ecological stability and sustainable resource management. Our results indicate that the oasis area expanded significantly (+2.04 x 103 km2 per decade), whereas water yield experienced a declining trend (-8.58 mm per decade). Notably, high water yield values remained concentrated in the adjacent mountainous regions, while the total water supply within oasis areas followed a "rise-then-fall" pattern, with 2010 marking a distinct turning point. Precipitation emerged as the primary determinant of water yield, and the combined influence of precipitation and temperature was found to be the most critical factor shaping its spatial distribution. The introduction of localized parameters into the InVEST model enhances the accuracy of water yield assessments in inland river basins. These findings underscore the importance of incorporating both oasis expansion and climatic variability into water resource management strategies, offering valuable insights for the sustainable evaluation and management of oasis ecosystem services in similar arid regions.