A novel long-wavelength turn-on fluorescent chemosensor CS based on pyrene was synthesized to detect Hg²⁺. In the presence of other metal ions, CS could effectively recognize Hg²⁺ and produce the turn-on fluorescent emission at 607 nm. Also, the absorption spectrum exhibited red-shift. Meanwhile, the change of the solution color from yellow to orange was directly observed by the naked eye. The interaction between CS and Hg²⁺ was confirmed by the Job’s plot, electrospray ionization mass spectrometry, scanning electron microscopy, and density functional theory calculations. It was found that the fluorescence of CS could be reversible when I^– was added into the solution of CS and Hg²⁺. CS illustrated high selectivity and good sensitivity for Hg²⁺ with the limit of detection of 36 nm. Moreover, CS could be utilized as test strips and silica gel plates to identify Hg²⁺.