In this work, we developed a red fluorescent turn-off Mg2+ probe with high Mg2+ selectivity. The quenching response to Mg2+ in combination with a green fluorescent turn-on Mg2+ probe enabled a ratiometric imaging of the changes in intracellular Mg2+ concentration. This ratiometric imaging system significantly enhanced the detection sensitivity of intracellular Mg2+ fluctuations in comparison with a conventional ratiometric Mg2+ sensing system, thus leading to a valuable technique for visualizing small change in intracellular Mg2+ concentration.