2-(6-hydroxynaphthalen-2-methylene) malononitrile (probe 1) was synthesized by means of the reaction of malononitrile and 6-hydroxy-2-naphthaldehyde, and the product was used as a fluorescent probe for the detection of fluoride ions. The results show that, in the detection system, the DMSO solution of probe 1 is pale yellow without fluorescence, and it immediately turns to be pink with blue-fluorescence after adding fluoride ions but shows no fluorescence after adding other anions. It is also found that probe 1 exhibits a strong selectivity and a high sensitivity to fluoride ions. According to the evaluation of the Job’s plot, the ratio of probe 1 to fluoride ions in the complex is found to be 1:1. The 1H NMR titration results confirm that the deprotonation of phenolic hydroxyl of probe 1 occurs after adding fluoride ions. The calculation results based on the Frontier molecular orbital theory indicate that, after the deprotonation of probe 1, its HOMO–LUMO energy gap increases, and the solution color and the fluorescence spectrum change dramatically.