Songnen Plain is one of the three great plains in northeast China with abundant groundwater resources. The continuous population growth and the rapid development of agriculture and economy in China has caused a series of environmental problems in the plain, such as endemic diseases caused by the accumulation of harmful substances in drinking water. This paper conducts a systematic investigation of fluorine in the groundwater of Songnen Plain. The results showed that fluorine was widespread in the groundwater of the plain in the concentration range of BDL–8.54 mg·L⁻¹, at a mean value of 0.63 mg·L⁻¹ and detectable at a rate of 85.91%. The highest concentrations of fluorine were found in central and southwest areas of the plain. The concentration exceeded the guideline values for fluorine in drinking water and may have varying degrees of adverse effects on adults, and especially children, in the study area. The fluorine in groundwater mainly came from the dissolution of fluorite and other fluorine-containing minerals, and the concentrations and distribution of fluorine were affected by cation exchange, groundwater flow field and hydrochemical indexes (pH, TDS and HCO₃⁻). The study provides scientific basis for the investigation, evaluation and prevention of endemic diseases caused by fluorine.