Hg²⁺ is one of the the most threatening heavy metal ions; thus, finding a quick and easy way to detect it is very important. In this study, we designed and synthesized a simple Hg²⁺ chemosensor Z, which bears carboxyl and imidazolyl moieties as the hydrophilic group site and Hg²⁺ binding site, whereas the naphthalimide moiety acts as the signal group. Interestingly, the sensor Z could dissolve in water and show high selectivity and sensitivity for Hg²⁺ in aqueous solution. Other coexistent competitive metal ions (Fe³⁺, Ag⁺, Ca²⁺, Cu²⁺, Co²⁺, Ni²⁺, Cd²⁺, Pb²⁺, Zn²⁺, Cr³⁺, and Mg²⁺) had no interference in the process of detecting Hg²⁺. The detection limit of sensor Z towards Hg²⁺ was 1.785 × 10⁻⁷ M, which indicated its high detection sensitivity. Moreover, the immediate response of Z to Hg²⁺ provides a real-time detection method for Hg²⁺ in aqueous solution. In addition, sensor Z can undergo cyclic utilization.