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Formaldehyde (FA) is widely used in industry and also common in daily life. Finding an efficient method to determine FA is quite an industrial challenge. Herein, a novel method based on a resonance light scattering (RLS) technique was developed for the detection of FA with high sensitivity. Carbon dots (CDs) were used as RLS probes. CDs were obtained via one-pot solvothermal treatment from o-phenylenediamine. CDs showed yellow fluorescence with a quantum yield of 0.41. Due to the multiple amino groups on the surface of CDs, FA can be captured easily by formation of a covalent C = N bond based on the Schiff-base reaction. Owing to the covalent crosslinking, CD nanoparticles aggregated, and even formed precipitate. The aggregation of CDs induced RLS enhancement, where the RLS increment was linearly related to the concentration of FA ranging from 4 nM to 1.6 mM, with a limit of detection (LOD) of 1.6 nM. In comparison with many previous reports, the present RLS method showed a wider linear range and lower LOD. Furthermore, the RLS system was successfully used to detect FA in real food samples. The proposed system has prospective applicability in the detection of FA in food fields.