<p>A rare earth complex was synthesized using α-thiophenoyltrifluoroacetone and o-phenanthroline as ligands. The complex was characterized by thermogravimetric analysis, infrared spectra, elemental analysis, and X-ray powder diffraction. The fluorescence spectra showed that the complex produced the characteristic fluorescence emission of Eu³⁺ ions at 615 nm, highly overlapping with the absorption peak of malachite green to cause fluorescence quenching. Fluorescence lifetime confirmed that the quenching mechanism is a fluorescence resonance energy transfer at low concentration, and fluorescence resonance energy transfer and inner filter effect at high concentration. The complex could exclude the interference of other aquaculture fish drugs and has a specific selectivity for malachite green. Response time was within 1 min, and the linear range was 0.1–70 μmol/L. Detection limit was calculated to be 0.023 μmol/L (3σ/k, n = 9). Spiked recoveries of malachite green in water and fish were in the range of 89.2–98.6%, indicating that the complex could be used as a fluorescent probe for the detection of malachite green residues.</p>