A rhodamine B-based off-on photochemical probe was developed for selective and sensitive mercury ion (Hg) detection. The probe features a simple synthesis route and demonstrates excellent dual-mode signaling through Hg-triggered colorimetric transition (colorless to pink) and fluorescence enhancement (580 nm). With low LODs of 4.4 μM (absorption spectroanalysis) 0.0023 μM (fluorescence analysis) and 2.6 μM (RGB analysis), and high anti-interference capability, this probe enables quantitative analysis and visual identification of Hg in multiple samples, such as environmental water, plant tissues (onion roots), food samples (rice, mung bean sprouts) and natural Chinese herbs (pueraria and honeysuckle). To facilitate field applications, the probe was engineered into low-cost test strips, exhibiting distinct Hg-responsive color/fluorescence changes in complex media (beverages, soil leachates). A smartphone-assisted quantification platform was established by correlating chromaticity ratios (R/B) with Hg concentrations. This integration eliminates reliance on bulky instrumentation, offering a portable solution for on-site Hg monitoring.