SBOD(sodium4,6dichlorophenolate) was designed and synthesized as a chalcone-based fluorescent turn-on chemosensor for Mg 2+ and Cd 2+ . SBOD selectively detected Mg 2+ and Cd 2+ through the increase in effective fluorescence. Detection limits of SBOD for Mg 2+ and Cd 2+ were calculated to be 3.8 μM and 2.9 μM, respectively. The binding modes of SBOD for Mg 2+ and Cd 2+ were determined to be 1:1 by ESI-MS and Job plot. Association mechanisms for SBOD to Mg 2+ and Cd 2+ were illustrated by ESI-MS, UV-vis, fluorescence spectroscopy, and calculations.