Accurate and sensitive detection of mercury ions (Hg) and thiram is essential for food safety monitoring and environmental protection. Here, an "on-off-on" fluorescent probe based on red-emitting carbon dots (CDs) was fabricated for sequential detection of Hg and thiram. The CDs exhibited red fluorescence at λ = 676 nm with a narrow full width at half maximum of 25 nm. Upon exposure to Hg, the fluorescence was significantly quenched due to the static quenching mechanism, achieving a limit of detection (LOD) of 25 nM. Notably, upon the addition of thiram, the quenched fluorescence of CDs was recovered due to the strong chelation between thiram and Hg, thereby enabling sensitive detection of thiram with a LOD of 4.80 nM. The approach was successfully validated using real water samples spiked with Hg and thiram. The recovery rates were between 97.16 % and 113.83 %, with relative standard deviations below 9.11 %. This study presents a simple, eco-friendly, and effective approach for the sensitive screening of trace contaminants in complex matrices.