Green, low-cost, and water-soluble fluorescent carbon dots were prepared under hydrothermal conditions, with a plant of Jinhua bergamot as a carbon source. The as-synthesized C-dots have better stability and relatively high photoluminescence with a quantum yield of 50.78%, along with the fluorescence lifetime of ca. 3.84 ns. Their photoluminescence can be significantly quenched by simply using the buffer solution of HAC–NaAC for Hg²⁺ and Tris–HCl for Fe³⁺. Based on this, we explored for selective detection of Hg²⁺ and Fe³⁺ with high selectivity, fast response, low cost, and broad linear ranges of 0.01–100 μM for Hg²⁺ and 0.025–100 μM for Fe³⁺, as well as low detection limits of 5.5 nM (Hg²⁺) and 0.075 μM (Fe³⁺) (S/N = 3).