Quick and effective detection of biothiols in biological fluids has gained increasing attention due to its vital biological functions. In this paper, a novel reversible fluorescence chemosensor ( L-Cu 2+) based on a benzocoumarin-Cu 2+ ensemble has been developed for the detection of biothiols (Cys, Hcy and GSH) in human urine. The chemosensing ensemble ( L-Cu 2+) contains a 2:1 stoichiometry structure between fluorescent ligand L and paramagnetic Cu 2+. L was found to exclusively bond with Cu 2+ ions accompanied with a dramatic fluorescence quenching maximum at 443 nm and an increase of an absorbance band centered at 378 nm. Then, the in situ generated fluorescence sluggish ensemble, L-Cu 2+, was successfully used as a chemosensor for the detection of biothiols with a fluorescence “OFF-ON” response modality. Upon the addition of biothiols, the decomplexation of L-Cu 2+ led to the liberation of the fluorescent ligand, L, resulting in the recovery of fluorescence and absorbance spectra. Studies revealed that L-Cu 2+ possesses simple synthesis, excellent stability, high sensitivity, reliability at a broad pH range and desired renewability (at least 5 times). The practical application of L-Cu 2+ was then demonstrated by the detection of biothiols in human urine sample.