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In the north arid and semiarid China, many sites have been suffering from high fluoride in drinking waters, such as the inland Tailan River Basin, where groundwater fluoride can reach 3.4 mg/L, compared to 1 mg/L of the Chinese standards for drinking water quality. In this paper, water chemistry and isotopes (H-2, O-18, H-3) were used to study hydrogeochemical evolution and fluoride accumulation. The results show that from river water to groundwater in the upper and lower part of the piedmont alluvial-proluvial plain, the main ions and fluoride shows systematic increase. The saturation index of fluorite seems to control the maximum fluoride content. Fluoride-minerals in the aquifers will continue to dissolve when fluorite is under saturated. Stable isotopes show that evaporation is not obvious during groundwater recharge and fluoride accumulation by evaporation concentration is limited. The high fluoride in groundwater is primarily caused by the protogenic geochemical processes in the basin.