Using blue-emitting UiO-66-NH₂ with red-emitting Eu-MOF as fluorescence groups, a ratio fluorescence probe (UiO-66-NH₂@Eu-MOF) was constructed and successfully applied to the determination of trace PO₄^3- ions in environmental water samples. A ratiometric fluorescent probe for the determination of trace PO₄^3- ions in environmental water samples was constructed via combining blue-emitting UiO-66-NH₂ with red-emitting Eu-MOF. As PO₄^3- ions concentration increases, the coupling of PO₄^3- ions with Zr⁴⁺ ions in UiO-66-NH₂ leads to a weakened ligand-metal charge transfer effect, effectively restoring the luminescence of UiO-66-NH₂. Also, an intense coordination of Eu³⁺ ions with PO₄^3- ions can hinder the ligand-metal energy transfer, destroy the "antenna" effect, and lead to the quenching of Eu-MOF fluorescence. Under optimal conditions, a ratio of the fluorescence intensity at 445 nm to the fluorescence intensity at 633 nm is linearly related to PO₄^3- ions concentration in the range of 1-12 μmol/L. The sensor constructed has a good selectivity and accuracy, thus providing an effective way for the rapid detection of PO₄^3- ions.