Abstract: Research on high-level radioactive waste (HLW) geological disposal in China has entered the stage of underground laboratory construction, with the Beishan preselected area in Gansu Province as the focal point. Groundwater redox potential (Eh) is a critical geochemical parameter governing the long-term safety of a repository. This study focuses on groundwater from Beishan Underground Research Laboratory construction area. Seven groundwater samples collected from the ramp and the 560 m deep drift during 2023-2025 were subjected to comprehensive chemical analysis. PHREEQC geochemical modeling was employed to back-calculate groundwater Eh, which were then compared with in-situ field measurements to systematically elucidate the spatiotemporal evolution of groundwater Eh under excavation-disturbed conditions. The results indicate that dissolved Fe is detected at significant concentrations in early-stage samples (2023-2024), whereas it is below detection limits in recent near-surface ramp groundwater. By contrast, Fe contents in groundwater from the 560 m deep drift were markedly higher than those in the near-surface ramp zone. Geochemical modeling reveals pronounced spatiotemporal heterogeneity in groundwater Eh: Early deep samples exhibit lower Eh, while late-stage shallow samples show higher Eh, with an overall trend of decreasing Eh with increasing burial depth and earlier sampling time. Dynamic field monitoring conducted in 2025 further demonstrated that groundwater in the 560 m deep drift is characterized by extremely low dissolved oxygen (DO, 0.03 mg/L) and low Eh (about 49.68 mV), whereas the ramp zone exhibits significantly higher DO and Eh. The simulated Eh for the 560 m deep drift groundwater (9.47-68.63 mV) agrees well with the field-measured value. Water-rock interaction experiments further corroborate the validity of the modeling results. In conclusion, this study validates the reliability of an Eh determination method combining Fe-based geochemical modeling with in-situ monitoring, confirming that deep groundwater at the Beishan Underground Research Laboratory maintains a reducing environment favorable for the long-term safe isolation of HLW. Nevertheless, the local groundwater redox environment is undergoing significant changes due to excavation disturbance. Under sealed in-situ conditions, actual Eh may be lower than those measured herein. This factor must be fully considered in the design of future in-situ experiments and safety assessments.