Abstract: The Dahei River is one of the most typical first-order tributaries of the Yellow River. The objective of this study was to investigate the groundwater-surface water sources and transformation in the mountainous, built-up, and irrigation areas of the Dahei River Basin. The recharge sources and hydrochemical mechanisms were also provided for the regional water resources and ecological protection. A total of 290 water samples (114 groundwater, 162 surface water, and 14 precipitation) were collected in April (dry season) and September (wet season) 2024. Some indicators were measured, such as the pH, total dissolved solids (TDS), major anions and cations (Na⁺, K⁺, Mg²⁺, Ca²⁺, HCO₃^-, SO₄^2-, Cl^-, and NO₃^-), δD, and δ¹⁸O. The chemical composition and hydrogen and oxygen stable isotopes of different water bodies were systematically analyzed using Gibbs diagrams, Piper trilinear diagrams, ion ratio methods, and MixSIAR models. The results revealed that the groundwater and surface water in the Dahei River Basin were generally weakly alkaline. The overall ionic concentration was ranked in the descending order of Na⁺＞Ca²⁺＞Mg²⁺＞K⁺ and HCO₃^-＞SO₄^2-＞Cl^-＞NO₃^-. Groundwater was dominated by the HCO₃-Ca∙Mg and HCO₃-Na∙Ca type, whereas the surface water was dominated by the HCO₃-Ca∙Mg type. The hydrochemical types of surface water also exhibited seasonal variations. The key ions of Na⁺, Ca²⁺, and HCO₃^- were dominant in the chemical composition of water bodies in the basin. There were diverse hydrochemical types of the groundwater and surface water, due to the influencing factors, such as leaching and human activities. The stable isotopes dD and d18O of surface water were more enriched during different periods, compared with groundwater. Both groundwater and surface water were primarily recharged by atmospheric precipitation. There was a significant seasonal variation. The precipitation dilution led to isotopic depletion during the wet season, while the evaporative concentration and snowmelt recharge resulted in the isotopic enrichment during the dry season. The MixSIAR model indicated that the main stream of the Dahei River and its first-order tributaries were primarily replenished by atmospheric precipitation (78.1% during the dry season and 85.2% during the wet season). In the mid- and downstream plain areas, the surface water infiltrated vertically through the vadose zone to replenish groundwater. In the midstream area, the contribution rates of surface water and precipitation to groundwater were 36.6% and 63.4% during the dry season, respectively, while 29.5% and 70.5% during the wet season, respectively. In the downstream area, the contribution rates were 31.6% and 68.4% during the dry season, respectively, while 26.9% and 73.1% during the wet season, respectively. The research findings can provide the scientific support for the zoned coordinated regulation of water quantity and quality in the Yellow River Basin. The upstream should further ensure the water conservation functions, while the midstream should implement groundwater extraction-recharge regulation, and the downstream should enhance the water use efficiency to optimize the irrigation quotas. The hydrochemical characterization was also integrated with the stable hydrogen-oxygen isotope tracing. Water source protection zones were scientifically determined for precision pollution prevention. The systematic solutions can also contribute to the water resource ecological protection and restoration in the basin.