A novel Schiff-base fluorescent sensor (HL) for Al³⁺ has been synthesized. The addition of Al³⁺ results in a significant increase of its fluorescent intensity at 420 nm in ethanol, however, other metal ions have almost no influence on the fluorescence. The reason for this phenomenon might be attributed to the formation of a 1 : 1 stoichiometric L–Al complex which inhibits the photo-induced electron transfer (PET) process. More importantly, the proposed binding mode of L–Al(III) is further identified by theoretical calculations.