Songnen Plain is controlled by geological structure, climate and other factors, and the fluorine content of groundwater in the region is generally high. In recent years, a huge number of researches have been put in practice based on high-fluorine groundwater, however, comprehensive understanding of the geochemical background in there is scarce, evolution trend and ecological health risks of fluorine in large reservoirs in the high-fluorine area of Songnen Plain likewise. Mechanisms of action and transport of fluorine ion (F-) in surface water of Xianghai Reservoir was analyzed using various methodologies, including mathematical statistics, the Piper three-line diagram, the Gibbs diagram, end-member analysis, PHREEQC hydrogeochemical simulations, as well as risk assessment models for ecological and health. Showing the results is that the concentration range of F- in the surface water of Xianghai Reservoir is 0.69 ~ 3.68 mg/L, and the average concentration is 1.87 mg/L, comparing with the concentration limit of 1.50 mg/L stipulated in China"s Surface Water quality Standard (GB 3838—2002) is higher, set by Class V. The surface water in this region is characterized by high fluorine content, with fluoride (F-) concentrations in the southwest significantly exceeding those found in the northeast. The hydrochemical characteristics of the regional surface water are primarily classified as HCO3-Na, HCO3-Na·Ca, and HCO3-Na·Mg. The main factors contributing to the enrichment of fluorine in Xianghai Reservoir include evaporation and concentration processes, mineral dissolution and precipitation, as well as cation exchange. Xianghai Reservoir belongs to the risk area of fluorine ecology and causes both children and adults facing health risks, with children being significantly more vulnerable to the harmful effects than adults.