Spirocyclization of xanthene dyes has become a powerful technique for developing fluorescent probes. Herein, we extend this unique fluorescence switching mechanism to a near-infrared (NIR) dye, 2-(7-diethylamino-2-oxo-2H-1-benzopyran-3-yl)-4-(2-carboxyphenyl)-7-diethylamino-1-benzopyrylium (CB), and construct a ratiometric fluorescent probe 1 for cysteine (Cys)/homocysteine (Hcy). The ratiometric sensing of probe 1 toward Cys/Hcy is realized by utilizing a tandem native chemical ligation/spirocyclization reaction to interrupt the large π-conjugated system of CB fluorophore, thereby affording remarkable blue shifts in the spectra of sensing system (from 669 to 423 nm in absorption spectra and from 694 to 474 nm in emission spectra). Probe 1 shows a high sensitivity for Cys/Hcy, and the detection limits (3 δ) for Cys and Hcy are 1.6 × 10(-7) and 1.8 × 10(-7) M, respectively. Moreover, since both the sulfhydril and the adjacent amino groups are involved in the sensing process, probe 1 is selective toward Cys/Hcy over other thiols such as glutathione. All these unique features make it particularly favorable for ratiometric Cys/Hcy sensing and bioimaging applications. It has been preliminarily used for Cys detection in rabbit serum samples and the ratiometric fluorescent imaging of Cys in living HepG2 cells.