Formaldehyde is usually a colorless aqueous solution or gas with a pungent odor, easily soluble in water, and irritating to human eyes and nose. In the presence of excessive ammonium salt, the reaction of formaldehyde and acetylacetone would generate a yellow compound, which has the maximum absorption at the wavelength of 414nm. The absorbance(A) is measured and the absorbance is stable within 3h of color development. And the standard operating curve is searched based on value A, and the concentration of formaldehyde is then calculated.