A new Hg2+-sensing and selective fluorescent sensor, 1-(1-pyrenecarboxaldehyd e)-thiocarbohydrazone, was synthesized and characterized by elemental analysis, FT-IR and 1H NMR spectra. Its fluorescence and recognition properties to the mercury ion were studied by the fluorescence emission spectra. With adding mercury ion into solution, the fluorescence emission intensities at different characteristic wavelength changed continually. Hence the ratiometric fluorescence measurement was used for detecting the complex process. It was found that a 1:1 stoichiometry complex is formed between the mercury ion and the compound with the association constants were 2.04×105 L/mol, respectively. And the detection limits of the mercury ion were 2.52×10-8 mol/L.