Abstract: This study examined the impact of vegetation restoration on water quality in small watersheds of the Loess Plateau since the implementation of the "Grain for Green" policy. The aim was to provide a reference for evaluating watershed water environment quality, optimizing water resource management, and guiding ecological construction and protection efforts. Using the Caijiachuan sub-watershed in the loess hilly and gully region of western Shanxi as the study area, long-term monitoring of surface water and groundwater quality was conducted in accordance with the China Ecosystem Research Network protocols for forest water environment monitoring. Analysis of variance (ANOVA) was employed to test for significant differences in water quality indicators between 2006 and 2023, while paired t-tests assessed seasonal variations between the flood and non-flood seasons. The results reveal the following: (1) The dominant anion in surface water in both 2006 and 2023 was HCO₃^-, with concentrations of 339.71 and 384.11 mg·L^-1, respectively. In groundwater, the dominant anions were HCO₃^- and SO₄^2-, with concentrations of 295.1 and 120.6 mg·L^-1 in 2006, and 351.22 and 343.89 mg·L^-1 in 2023, respectively. The dominant cation in both water types in 2006 was Ca²⁺, at 46.47 mg·L^-1 in surface water and 92.2 mg·L^-1 in groundwater. In 2023, Ca²⁺ remained the dominant cation in groundwater (82.97 mg·L^-1), while Na⁺ became dominant in surface water (56.11 mg·L^-1). (2) Between 2006 and 2023, Mg²⁺ and Cl^- concentrations in surface water increased by 74.63% and 97.78%, respectively, while K⁺ decreased by 73.50%. In groundwater, concentrations of Mg²⁺, Na⁺, SO₄^2-, and NO₃^- increased by 228.77%, 44.62%, 185.15%, and 353.64%, respectively, whereas K⁺ decreased by 26.67%. (3) In 2006, surface water TDS and HCO₃^- concentrations, along with groundwater SO₄^2- concentration and pH, were significantly higher in the non-flood season than in the flood season (P < 0.05). In 2023, surface water COD and groundwater Ca²⁺ concentrations were significantly elevated during the non-flood season (P < 0.05). Overall, vegetation restoration improved the water quality of both surface water and groundwater in the Caijiachuan sub-watershed. However, subsequent anthropogenic activities have introduced certain pollutants, indicating the need for continued environmental management. These findings provide a scientific basis for the integrated management of water environment in watershed.