A target-responsive ratiometric fluorimetric sensing strategy for Hg2+ has been rationally designed. The sensing probe was established based on a functionalized metal-organic framework, which was prepared with 3,5-dicarboxyphenylboronic acid (DCPB) as the functional ligand and Eu3+ as the metal junction. The porous nano-spheres of Eu-MOF with an arylboronic acid as the functional recognition group for Hg2+ exhibited tunable optical properties with dual emission fluorescence signals at 338 nm and 615 nm. In the presence of Hg2+, arylmercury was formed by a specific transmetalation reaction between Hg2+ and arylboronic acid groups, which blocks the energy transfer between the ligand and Eu3+. Thereby, the fluorescence signal of Eu-MOF/BA at 615 nm decreased, while the fluorescence signal at 338 nm remained almost constant. The ratiometric fluorimetric sensing for Hg2+ was achieved by calculating the peak intensity ratio of F615/F338 based on the reference signal at 338 nm and the response signal at 615 nm. The detection limit of Hg2+ was as low as 0.0890 nM, and the recovery rate of the actual environmental water sample ranged from 90.92% to 118.50%. Therefore, the excellent performance of the ratiometric fluorimetric sensing method for Hg2+ makes it attractive for the detection of heavy metal ions in environmental monitoring.