The understanding of F- concentration in groundwater in humid areas is limited although there are lots of research on high-fluoride groundwater in arid areas. In this paper, with controlling factors of F- concentrations in humid areas as the focus, 130 groundwater samples, obtained from four subsystems in Northwest Xingguo County, Jiangxi Province, China, were investigated to demonstrate the controlling factors of F- concentrations in humid areas. According to analytical results, the following hydrogeochemical characteristics of the fluorine in humid mountainous areas were determined: (1) F- concentration is positively correlated with total dissolved solids (TDS), Ca2+, HCO3-, and pH; (2) the groundwater features a high flow rate and low TDS; (3) the equilibrium constant of CaF2 is less than its solubility product constant, and the fluorine-bearing minerals in rocks are in a dissolved state; and (4) the dissolved fluoride-bearing minerals constitute the main sources of F- in the groundwater. Fluorine mainly comes from groundwater fluorine-bearing minerals in metamorphic rocks. Moreover, the low F- concentration in the groundwater mainly results from the fast flow rate of groundwater. Fluoride in groundwater has great potential hazards in humid areas.