The invention discloses a testing agent for implementing field fast detection of formaldehyde in the air and relates to a reagent for content determination, the testing agent consists of acetylacetone source water solution and an acetylacetone releasing agent which are respectively packaged, the acetylacetone source water solution comprises acetylacetone and soluble salts containing colorless metal ions, and the acetylacetone releasing agent comprises a metal ion high-stable chelating agent and sparingly volatile weak acid; and the acetylacetone source water solution or the acetylacetone releasing agent further comprises an ammonia source. When the testing agent is used for field detection of the formaldehyde, 0.5ml of the acetylacetone source and 0.5ml of the acetylacetone releasing agent are respectively added in the detected solution, the acetylacetone releasing agent can release acetylacetone from the acetylacetone source, the color is further developed by being reacted with the formaldehyde and the ammonia in the weak acid medium, and then the result can be fast obtained on the field.