Integrating covalent organic framework (COF)–metal organic framework (MOF) hybrid materials is an effective way to develop a ratiometric fluorescence method. Herein, we report a strategy for the growth of lanthanide organic frameworks on the surface of a COF. A carboxyl-functional fluorescent COF was first synthesized under solvothermal conditions. The surface –COOH reacts with a europium (Eu³⁺) ion and a ligand to integrate the MOF on the COF. The prepared COF–MOF shows high thermal stability and chemical stability. Moreover, the COF–MOF exhibited dual emissions at 446 nm and 621 nm, which shows the fluorescence resonance energy transfer (FRET) process from the COF to MOF. With dual emissions, structural stability and phosphate affinity, the COF–MOF based sensing strategy results in good performance for the ratiometric fluorescence detection of phosphate.