The invention relates to a monoamine oxidase diagnostic/measuring reagent (kit) which utilizes enzyme colorimetry and enzyme-linked method technologies, and meanwhile, the invention also relates to a method for measuring monoamine oxidase active concentration and compositions and components of the reagent, which belong to the technical field of medical inspection and measurement. The main components of the reagent (kit) of the invention comprise a buffer solution, coenzyme, an amine compound, glutathione disulfide, glutathione oxidase, glutathione-disulfide reductase and a stabilizer; and the method comprises the following steps of: enabling a sample and the reagent to generate a series of enzymatic reactions through mixing the sample and the reagent according to a certain volume ratio, then placing the reactants under an ultraviolet/visible light analyzer and detecting the rising speed of the absorbance at a position with the dominant wavelength of 340nm to measure and calculate the active concentration magnitude of monoamine oxidase.