A novel fluorescent probe was synthesized and explored to detect cadmium ions (Cd²⁺) based on an imine functionalized metal–organic framework (MOF). The nanoscale MOF probe (∼100 nm) displayed a turn-on fluorescence in aqueous environments with the addition of Cd²⁺ which could be ascribed to the interaction between the CN group and specific metal ions, presenting a low-toxicity, highly selective and sensitive detection method for Cd²⁺ with a detection limit of 0.336 μM (37.8 ppb) and a broad linear range of 0–500 μM, which is promising for application in intracellular sensing and imaging of Cd²⁺. The probing mechanism was studied and discussed in detail.