Cellulose triacetate (CTA) was first applied as the catalytic support to load Pt nanoparticles for low-temperature formaldehyde (HCHO) decomposition. The room-temperature HCHO decomposition rate of the obtained catalyst (Pt/CTA) is 13.4 times and 4.3 times as high as that of the microcrystalline cellulose-supported Pt catalyst and Pt/TiO2 under the parallel preparation condition, respectively. With facile shaping, the CTA microsphere-supported Pt catalyst and the CTA film-supported Pt catalyst could also exhibit similar HCHO decomposition performance to that of the powdery one. Structural analyses showed that Pt nanoparticles (similar to 2.3 nm) could densely disperse on the small-area surface of Pt/CTA and provide abundant active sites. Moreover, only the HCHO molecules could slightly adsorb onto CTA, while other HCHO decomposition-related species absolutely could not. This is beneficial to the coordination of various steps of HCHO decomposition and the transfer of reaction species to vicinal active sites of Pt/CTA. HCHO-diffuse reflectance infrared Fourier transformed spectroscopy studies demonstrated that no species were accumulated on the Pt/CTA catalyst. Both the good Pt dispersion and unique adsorption properties of CTA were responsible for the excellent low-temperature HCHO decomposition performance of Pt/CTA.