The invention relates to a detection reagent for fast detecting cyanide in water. The detection reagent comprises a release reagent and a color developing reagent, wherein the release reagent comprises monopotassium phosphate, methylcellulose, lactic acid and acetone mixed solvents and chloramine T, the color developing reagent comprises sodium hydroxide, isonicotinic acid, barbituric acid and methylcellulose, and the release reagent and the color developing reagent are encapsulated in a separated way according to the consumption for each time. The release reagent comprises the preparation steps that: 1, major ingredients and deionized water are mixed and ground into paste; 2, the paste is placed in a vacuum drying box for drying; 3, dried materials are cooled and ground into powder; 4, the lactic acid and acetone mixed solvents are used for preparing 5% chloramine T solution; and 5, the chloramine T solution is added into the prepared powder, the materials are dried in the air, and the powdery release reagent is obtained. The preparation steps of the release reagent refer to the partial steps. The detection reagent disclosed by the invention can be matched with a colorimetric tube and a standard colorimetric card to be used, the carrying is convenient, the detection range is wide, the time is short, and convenience can be provided for the fast detection of the cyanide in water in various sites.