The monitoring of mercury and fluoride ions (Hg2+ and F-) has aroused wide concerns owing to the high toxicity of Hg2+ and the duplicitous nature of F- to human health. As far as we known, more than 100 million people in poverty-stricken areas are still at high risk of being over-exposed to Hg2+ and F- via drinking water. Simple and cost-effective luminescent methods are highly promising for on-site water monitoring in rural areas. However, the development of multipurpose luminescent probes that are accurate and sensitive remains challenging. Herein, a new strategy for rationally designing a multipurpose ratiometric probe is present. The obtained probe is consisted of two emission units with energy transfer between them, which exhibit high coordination affinities to the two coexisted toxic targets (Hg2+ and F-), respectively. Thus, two distinct routes for efficiently modulating the energy transfer in the probe are present to trigger the responses to the two targets in sequence. By detecting the shift of the emission color with a smartphone, an on-site water monitoring method is successfully established with the detection limits as low as 2.7 nM for Hg2+ and 1.9 μM for F-. The present study can expend the toolbox for water monitoring in rural regions.