We report a metal-organic frameworks (MOFs)-based nanocomposite by incorporating a fluorescent dye, rhodamine B (RhB), into a stable Zr-based MOF, UiO-66-NH2. Based on weakened the LMCT process resulted from the strong binding of phosphate with Zr-O node in UiO-66-NH2 and the unchangable fluorescence of RhB as the reference, this nanocomposite can be employed as a novel fluorescent sensing platform for ratiometric detection of phosphate in the range from 80 to 400 mu M with a detection limit of 2.0 mu M. It is particularly suitable for accurate quantification of phosphate in complex samples like human serum. Furthermore, this sensing platform is well recyclable due to reversible interaction with phosphate. Finally, this nanocomposite was encapsulated into agarose hydrogels for direct detection of phosphate in human serum with the naked eye. Thanks to the robust, portable, and instrument-free characteristics, it is expected to show great potential for on-site clinical diagnosis.