Mercury (Hg) and its derivatives pose a serious threat to the environment and human health because of their durability, easy transference, and high biological accumulation. Thus, the development of methods for selective and sensitive determination of He2+ is very important to understand its distribution and implement more detailed toxicological studies. Herein, we report a simple 4-hydroxynaphthalimide-derived fluorescent probe (1) for the detection of both inorganic and organic mercury species in aqueous solution. Probe 1 could quantificationally detect mercury species by fluorescence spectroscopy with high selectivity and sensitivity. Importantly, probe 1 could serve as a "naked-eye" indicator for mercury species and exhibited a large fluorescent enhancement with the help of hydrogen peroxide (H2O2. Additionally, the mechanism of the reaction between probe 1 and mercury species was confirmed using H-1 NMR and ESI-MS. And, analytical applications of probe 1 to the river water samples further demonstrated that it provided an excellent method for the determination of mercury species in the environment.