Vegetation restoration can improve regional climatic suitability and enhance ecosystem stability. However, the increased water consumption resulting from vegetation greening has raised concerns about water resource availability. Therefore, developing a scientifically sound framework for water suitability assessment is crucial to achieving sustainable water resource management. Based on this, the study focused on the Sanchuan River Watershed, a typical vegetation restoration area on the Loess Plateau. Using the Regional Hydro-ecological Simulation System (RHESSys) model as technical support, a comprehensive evaluation framework for socionatural water suitability was developed. The spatial and temporal characteristics of regional water suitability from 1990 to 2020 were analyzed across three dimensions: social (Ms), natural (Mn), and socio-natural (Msn). Furthermore, the key driving factors and underlying mechanisms influencing changes in water suitability were systematically investigated. The results showed that the southeastern region had lower Mn values (0.46-1.00), while areas with lower Ms values (0.70-1.00) was located in the central region, where human activities were more intensive. Consequently, Msn (0.78-2.00) exhibited relatively lower levels across the southeastern and central regions of the watershed. Temporally, after 1999, the interannual trends of Ms, Mn, and Msn changed from decreasing to gradually increasing. Among the different land-use types, grassland exhibited the highest Msn value (1.75), which was better than shrub (1.70) and forest (1.63), indicating that grassland has relatively better water suitability. The results of the structural equation model (SEM) showed that socio-economic factors exerted significant direct negative effects on Ms (-0.56), Mn (-0.97), and Msn (-0.98), while vegetation, meteorological, and topographic factors indirectly affected each water suitability dimension through multiple pathways. The study results provide a reference for sustainable water resource management in ecological restoration regions.