A new compound, ethyl 5-phenyl-2-(p-tolyl)-2H-1,2,3-triazole-4-carboxylate was successfully introduced and synthesized as a novel rhodamine B derivative named REPPC, and characterized by H nuclear magnetic resonance (NMR), C NMR, and high resolution mass spectrometry (HRMS). It showed an obvious fluorescence and UV-visible light absorption enhancement towards Hg ion without interference from common metal ions in N,N-dimethylformamide-H O (pH 7.4). The spirolactam ring moiety of rhodamine in REPPC was converted to the open-ring form generating a 1:1 complex with the intervention of a mercury ion, verified by electrospray ionization-mass spectroscopy testing and density functional theory calculation. REPPC was used to visualize the level of mercury ions in living HeLa cells with encouraging results.