The invention relates to a fluorescent probe, wherein, the fluorescence is enhanced in the presence of ONOO-, and the fluorescence returns to original state in the presence of reducing matter. The invention provides a reversible fluorescent probe for selective detection of ONOO- in cells, characterized in that: cyanine dye is used as the fluorescent matrix, an organic selenide structure is introduced in the cyanine matrix as the active site of the reaction with ONOO- to realize the selective detection of ONOO-; the properties that organic selenium oxide formed by oxidizing organic selenide is easy to be reduced to organic selenide by reducing micromolecules in biosystem, such as cysteine, reducing glutathione, metallothionein and the like, is utilized to realize the reversibility of the probe molecules; and variations of electronic properties of organic selenide before and after the oxidation and the influence of the variations on the fluorescence property of the whole compound are utilized simultaneously to modulate the fluorescence property of the probe molecules.