In this paper, we prepared a new probe, a supramolecular compound composed of esculetin (ESC) and a cucurbit[8]uril (Q[8]). Ultraviolet, fluorescence and other methods have shown that ESC and Q[8] formed a 2 : 1 (ESC₂@Q[8]) inclusion compound with weak fluorescence, which could efficiently identify metal aluminum ions in a pH = 5.0 water system, and emit strong indigo blue fluorescence to achieve naked eye recognition. In the competition experiment, this probe exhibited the specific recognition of aluminum ions. Furthermore, its recognition mechanism was explored through crystal diffraction and it was found that the carbonyl oxygens on the port of Q[8] and the hydroxyl oxygen atoms of ESC together formed a ternary complex with Al³⁺ ((ESC₂@Q[8])@Al³⁺), which made the host–guest fluorescent probe form a hyper-conjugated system to achieve the effect of fluorescence enhancement. The fluorescence intensity had a good linear correlation with Al³⁺ concentration (R² = 0.9838) in the range of 0.3–2.4 × 10⁻⁵ mol L⁻¹. The detection limit was 9.39 × 10⁻⁹ mol L⁻¹, which may have potential and application prospects for detecting metal ions in water systems.