A new fluorescent sensor for the selective and sensitive sensing of Cu²⁺ and Fe³⁺ based on the rational control of multiple mechanisms has been developed. Sensor L is weakly-fluorescent (ϕ = 0.029) due to the CN isomerization and PET process. Upon binding with Cu²⁺, the complex L + Cu²⁺ can give a remarkable fluorescence enhancement (ϕ = 0.182) with a limit of detection down to 1.3 × 10⁻⁸ mol L⁻¹ because of both inhibition of the CN isomerization and PET process. However, upon binding with Fe³⁺, the original weak fluorescence of L is completely quenched (ϕ = 0.002) due to the paramagnetic quenching property. In addition, the complex L + Cu²⁺ could be further used as a new platform for the ultrasensitive detection of Fe³⁺ based on the fluorescence “turn-off” response with a limit of detection down to 2.4 × 10⁻⁸ mol L⁻¹ in aqueous solution.