The invention provides a preparation method and application of a composition for decomposing low-molecular aldehyde without illumination. The composition mainly comprises chitin, EDTA, carbohydrazide, glycerin, polyethylene glycol, dicyandiamide, sodium phytate, phytic acid, glucoside, sodium sulfanilate, mannitol, citric acid, sodium thiosulfate, modified starch, cellulose, gelatine powder, aluminum oxide, silica, diatomite and kaolin. The composition can be used for removing formaldehyde, acetaldehyde and other low-molecular aldehyde and harmful small polar molecules in gas-liquid-solid phase substances. The composition can be used for removing residual formaldehyde in decorative materials in buildings, and an aqueous solution and the gas state of the composition can be used for decomposing formaldehyde in the liquid phase and the gas phase. Solid of the composition can be added into materials such as indoor building materials, wallpaper glue, diatom ooze, seashell ooze, silicon balls, diatom balls, interior wall coating materials and water-based paint. According to a decomposing function of the composition, one part of the composition with the components can decompose 2 parts of formaldehyde solutions (40%), salt and water are generated, secondary pollution is avoided, illumination is of no need, and the formaldehyde decomposition rate can reach 99.7% or over.