[Objective] This study aims to reveal the spatio-temporal distribution patterns and driving mechanisms of fluoride in the wetland water of Xianghai National Nature Reserve, clarify the reasons for water quality exceeding standards, and provide theoretical support for the scientific formulation of ecological governance measures for Xianghai Wetland, as well as offer experience for regional water environment governance and improvement of water quality in the basin. [Methods] Based on the long-term monitoring data of the national control section of surface water in Xianghai Nature Reserve from 2012 to 2021, the different concentration characteristics of fluoride in the flood season, normal water season and dry season were compared and analyzed to clarify the inter-annual and seasonal variation characteristics of fluoride. Pearson correlation analysis was used to explore the correlations among various water quality indicators, and Gibbs diagram was combined to analyze the water chemical characteristics of surface water and interpret the natural sources of fluoride. [Results]① From 2012 to 2021, the annual average fluoride concentration in the surface water of Xianghai Reservoir ranged from 2.02 to 3.87 mg/L, chronically exceeding the Class V surface water quality standard (GB3838-2002) by 0.36–1.61 times annually.② Temporally, fluoride concentrations were higher in the dry season (3.54–3.66 mg/L) and lower in the wet season (2.76–2.87 mg/L), exhibiting an initial decline, subsequent rebound, and final decline with pronounced seasonal fluctuations. Spatially, concentrations at Yichangpao (1.56–3.2 mg/L, 2022) significantly exceeded those at Erchangpao (1.16–2.48 mg/L).③ Sediment total fluoride content showed distinct spatial heterogeneity, with mean values at Yichangpao (576 mg/kg) far exceeding Erchangpao (113 mg/kg), aligning with water-soluble fluoride distribution trends. Surrounding soil exhibited high total fluoride levels.④ The dominant hydrochemical types of surface water were HCO??—Na?Mg and HCO??—Na, with fluoride enrichment closely linked to regional evaporation concentration, dissolution-precipitation, and ion exchange processes. [Conclusion] Endogenous release is the primary pollution pathway for fluoride in Xianghai Reservoir’s surface water. Control measures should focus on endogenous pollution management and hydrological regulation.