Abstract: The objective of this study was to investigate the groundwater-surface water transformation processes in the mountainous area, built-up area, and irrigation area of the Dahei River Basin, a typical first-order tributary of the Yellow River. The research aimed to clarify the recharge sources and hydrochemical mechanisms, providing scientific basis for regional water resources management 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, pH, total dissolved solids (TDS), major anions and cations(Na⁺、K⁺、Mg²⁺、Ca²⁺、HCO—3、SO2− 4、Cl⁻、NO—3), δD, and δ¹⁸O were measured. The chemical composition and hydrogen and oxygen stable isotope characteristics of different water bodies in the Dahei River Basin 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 are generally weakly alkaline, with the overall ionic concentration showing Na⁺＞Ca²⁺＞Mg²⁺＞K⁺ and HCO—3＞SO2—4＞Cl^—＞NO—3. Groundwater is dominated by the HCO₃-Ca∙Mg type and HCO₃-Na∙Ca type, surface water is dominated by the HCO₃-Ca∙Mg type. The hydrochemical types of surface water exhibit distinct seasonal variations, with Na⁺, Ca²⁺, and HCO—3 been the key ions controlling the chemical composition of water bodies in the basin. However, due to factors such as leaching effects and human activities, the hydrochemical types of both groundwater and surface water demonstrate diverse characteristics. The stable isotopes δD and δ¹⁸O of surface water in the basin are more enriched compared to groundwater during different periods, with both groundwater and surface water primarily recharged by atmospheric precipitation. However, there existed significant seasonal variation: during the wet season, precipitation dilution lead to isotopic depletion, while during the dry season, the combined effects of evaporative concentration and snowmelt recharged result in isotopic enrichment. The MixSIAR model indicates that the main stream of the Dahei River and its first-order tributaries are 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, surface water infiltrates vertically through the vadose zone to replenish groundwater. In the midstream area, the contribution rates of surface water and precipitation to groundwater are 36.6% and 63.4% during the dry season, and 29.5% and 70.5% during the wet season, respectively. In the downstream area, the contribution rates are 31.6% and 68.4% during the dry season, and 26.9% and 73.1% during the wet season. The research findings provide scientific support for zoned coordinated regulation of water quantity and quality in the Yellow River Basin: the upstream should further ensure water conservation functions, the midstream should implement groundwater extraction-recharge regulation, and the downstream should enhance water use efficiency by optimizing irrigation quotas. By integrating hydrochemical characterization analysis with stable hydrogen-oxygen isotope tracing technology, water source protection zones are scientifically demarcated, and a precision pollution prevention and control system is established, providing systematic solutions for unified water resource management and ecological protection and restoration in the basin.