The invention provides a method for detecting iodide ions by an unmarked fluorescent probe based on thioflavin T dye design. The method comprises the steps of adding a buffer solution into the prepared thioflavin T/deoxyribonucleic acid (DNA)/ Hg<2+> unmarked fluorescent probe, and adding iodine ion solutions with different concentrations into reaction liquid for carrying out a reaction; measuring a fluorescence value of the mixed liquid, and establishing an equation for detecting the solubility of the iodide ions in unknown liquid under test. After the method is used, the currently common detection mode for detecting the iodide ions is changed, and the iodide ions are detected by an unmarked fluorescent probe technology for the first time, so that the defects that the existing detection method is low in sensitivity and complicated in operating steps, needs expensive large-scale instruments, is difficult in realization of rapid on-line detection, and the like can be overcome; the method has the advantages of being rapid in detection response, high in sensitivity, good in selectivity and the like, is a proper, effective and practical detection method, and can be applied to determination of the iodide ions in practical samples.