Abstract: In order to identify the evolution trend and reveal the reasons for the abnormally high fluoride concentration in Hulun Lake, field surveys of the fluoride concentration in the inflow rivers, surrounding groundwater and lake water were conducted from 2015 to 2019. Together with the historical fluoride concentration in Hulun Lake from 2005 to 2014, the factors influencing the fluoride concentration in Hulun Lake were analyzed. Based on the data from the whole-lake surveys conducted in 4 different seasons of 2018-2019, the whole-lake average fluoride concentration in Hulun Lake was 2.27-2.42 mg/L, with an annual average of 2.36 mg/L, and there was no significant difference between the average concentrations of the four seasons. However, the lake water fluoride concentration showed different spatial distribution patterns in the four seasons. While the fluoride concentration in the areas near the lakeshore was lower than in the lake center in spring, summer and autumn, the fluoride concentration was higher near the lakeshore in winter. The fluoride concentrations in the water of the three major inflow rivers of Hulun Lake were (1.14±0.36), (0.84±0.14) and (0.33±0.08)mg/L, which were significantly lower than that of the lake water. The fluoride fluxes from the three major rivers into the lake were 236.41, 396.31 and 301.29 t/a, respectively. Inflow rivers and groundwater discharge were the main input sources of fluoride in Hulun Lake. The fluoride concentration in the water of Hulun Lake was mainly resulted from the high natural background environment caused by the special climatic and geographical conditions, and it was also affected by pH, lake water volume and icing. Accumulation and concentration of the fluoride from the inflow rivers, groundwater discharge and other sources in the lake under the influence of the high evaporation and a lack of fluoride output are the primary causes of the high fluoride concentration in the Hulun Lake water.