<p>Formaldehyde is often illegally used as a preservative in aquatic products to extend shelf life, posing severe threats to public health due to its carcinogenicity. Herein, amino-functionalized carbon dots (CDs) were prepared via a simple one-pot hydrothermal approach employing dicyandiamide and sodium citrate as starting materials, and a rapid, selective fluorescent quenching-based method for formaldehyde detection was established. The prepared CDs exhibited remarkable solubility in water, a high fluorescence quantum yield of 36.3%, and strong resistance to salt-induced aggregation, photobleaching, and long-term storage. Based on the Schiff base reaction between the amino groups on the surface of CDs and formaldehyde, a rapid and selective fluorescence quenching method was developed for formaldehyde detection in aqueous media. The detection mechanism was systematically investigated through spectroscopic studies and density functional theory (DFT) simulation calculations, demonstrating that imine formation significantly suppresses the electron transfer process from the amino groups to the carbon core, leading to effective fluorescence quenching. Under optimized conditions, the method exhibited a linear response within the concentration range 50-1000 µM, with a detection limit (LOD) of 24.2 µM. The probe exhibited high selectivity in the presence of various common interfering substances and was effectively utilized for formaldehyde determination in grass carp samples, yielding satisfactory recoveries ranging from 91.6% to 108.1%. This study presents a straightforward, fast, and dependable approach for formaldehyde monitoring in aquatic products, holding promise for food safety supervision and on-site detection applications.</p> Graphical Abstract <p></p>