ABSTRACT: Developing a specific, ratiometric, and reversible detection method for metal ions is significant to guard against the threat of metal-caused environmental pollution and organi**s poisoning. Here a supramolecular host−guest system (WP5⊃G) based on water-soluble pillar[5]arene (WP5) and water-soluble quaternized perylene diimide derivative (G) was constructed. Morphological transformation was achieved during the process of adding WP5 into G aqueous solution, and a fluorescence “turn-off” phenomenon was observed which was caused by supramolecular photoinduced electron transfer (PET). Meanwhile, hydrophobic effect and electrostatic interaction played important roles in this supramolecular process, which was confirmed by isothermal titration calorimeter (ITC) and ζ potential experiments. Furthermore, the supramolecular host−guest system could be a “turn-on” fluorescent probe for Fe3+ ion detection through the process of interdicting supramolecular PET. Moreover, the Fe3+ ion detection showed specific, ratiometric, and reversible performances with a detection limit of 2.13 × 10−7 M, which might have great potentials in biological and environmental monitoring.