The selective and sensitive detection methods for toxic transition-metal ions, which are rapid, facile, and applicable to the environmental and biological milieus, are of great importance. In this study, we designed a beta-CD-based ratiometric sensor for detecting mercury ions in aqueous media, some biological fluids, and live cells. In this sensing platform, the thiocarbamido-containing probe dye was covalently linked onto the hydrophilic beta-CD rim, which is conducive to complexing with metal ion, while the donor dye was anchored inside hydrophobic beta-CD cavity via the adamantyl Moiety. which is good for avoiding sell-aggregation and enhancing the quantum yield of the donor dye. Upon associating with mercury ion, the probe dye undergoes ring-opening process and serves as the energy acceptor and constitutes the FRET system with the donor dye; by this way ratiometric detection of mercury ion in water can be realized with the detection limit of 10 nM. The cyclodextrin plays a crucial role for the sensing system; it not only accommodates both the donor dye and the probe dye which can form FRET system upon addition of Hg2+ but also makes the sensor water-soluble and cell membrane permeable. This nontoxic sensing platform can be used for mercury ion detection in aqueous medium. biological fluids, and live cells (L929 and Hela). We also found that. upon being taken up by L929 cells, the sensor exhibited no cytotoxicity, and the cell proliferation was not affected.