Study region: The middle reaches of the Heihe River Basin, an arid inland river system in northwest China, where groundwater plays a key role in sustaining oasis ecosystems under strong evaporative conditions. Study focus: This study investigated groundwater hydrochemical characteristics and their linkages with vegetation dynamics using major ions (Na+, K+, Ca2+, Mg2+, Cl-, SO42-, HCO3-, NO3-) and physicochemical parameters (pH, EC, TDS). Multivariate statistical methods, including K-means clustering and principal component analysis (PCA), were applied to identify hydrochemical types and controlling processes. Relationships among groundwater chemistry, normalized difference vegetation index (NDVI), and potential evapotranspiration (ET0) were analyzed to explore ecohydrological interactions. New hydrological insights for the region: Groundwater is weakly alkaline and dominated by Na+ and HCO3-, mainly controlled by mineral dissolution, evaporation-concentration, and anthropogenic inputs. A clear salinity gradient was observed, with higher TDS in northern irrigated areas. Strong ecohydrological coupling is evident, as evaporation-driven salinity accumulation (positive ET0-TDS relationship) constrains vegetation growth. NDVI shows negative correlations with TDS and major ions, indicating salinity stress. Notably, elevated HCO3- further suppresses vegetation, suggesting alkalization as an additional ecological constraint. These findings provide a scientific basis for salinity management and ecosystem protection in arid regions.