Abstract: This study successfully developed a highly sensitive formaldehyde detection method based on NaYF₄-CDs fluorescent composite nanomaterials. This method innovatively utilizes amide bonds to covalently modify carbon dots (CDs) on the surface of upconversion nanoparticles (NaYF₄:Yb³⁺/Er³⁺), constructing a novel composite nanomaterial. This material achieves specific detection of formaldehyde by inducing fluorescence quenching through the reaction of amino groups on the surface of CDs with Schiff base of formaldehyde. The introduction of NaYF₄ nanoparticles not only increases the spatial distance between carbon dots, effectively reducing concentration quenching effects, but also significantly improves material stability. The experimental results show that NaYF₄-CDs exhibit good linear response in the concentration range of 1 nmol/L to 700 nmol/L, and can achieve high-sensitivity detection of formaldehyde at the nanogram level. This material combines the advantages of high stability, excellent selectivity, easy operation, and low cost, providing an innovative solution for accurate and rapid detection of formaldehyde in complex environments.