We have synthesized rhodol hydrazide (RDH) as a simple fluorescent probe for detecting Hg²⁺. The probe can be applied in nontoxic solvents (EtOH and H₂O). The probe has high selectivity and sensitivity to Hg²⁺ at pH 6–8. In addition, the probe has a superior capacity to resist interference from other ions. Both fluorescence intensity and absorbance have a linear relationship with the concentration of Hg²⁺, which ensured the precise detection of Hg²⁺. Furthermore, we have studied the intracellular Hg²⁺ imaging behavior of the probe on mammalian cells, which indicated that the probe can be applied to monitor Hg²⁺ within biological samples, especially in mammalian cells.