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A novel recyclable ratiometric fluorescent probe based on MOFs was designed and synthesized by a one-step approach, which was realized by assembling Al-MOFs in an aqueous solution containing rhodamine B (RhB) as a fluorescent reagent and Fe3O4 as a magnetically recyclable reagent. The obtained fluorescent probe (Fe3O4/RhB@Al-MOFs) has two emissions under excitation at 330 nm, in which the emission at 440 nm is attributed to the Al-MOFs and the emission at 610 nm presumably originates from the RhB dye. It was noted that the probe displays responses to Mg2+ ions with fluorescence enhancement at 440 nm, whereas the fluorescence at 610 nm would be a constant reference fluorescence signal. The existence of RhB significantly improves the detection accuracy and broadens the linear range. As a result, the selectivity of Fe3O4/RhB@Al-MOFs towards Mg2+ compared to other metal ions is excellent. When the Mg2+ ions (262.5 ppm) are added, the ratio of fluorescence intensity (I440/I610) is changed from 2.47 to 11.5. The Fe3O4/RhB@Al-MOFs show a rapid detection process (90 s), high fluorescence enhancement (up to 5-fold) and a low detection limit of Mg2+ ions (8 × 10−7 M). In particular, the magnetic Fe3O4 endows Fe3O4/RhB@Al-MOFs with a high magnetic responsiveness, thus, facilitating the separation procedures. As expected, it can be easily separated and recycled without a significant loss of recognition performance after being used many times, and thus, exhibits good reusability. Meanwhile, the cell cytotoxicity experiments suggest that Fe3O4/RhB@Al-MOFs is nearly non-toxic and in vitro cell imaging experiments demonstrate that the probe is capable of successfully enhancing fluorescence from the intracellular area in HL-7702 and A375 cellular systems. Hence, it has the potential for the detection of Mg2+ ions inside biosystems. [ABSTRACT FROM AUTHOR]