The invention relates to a lactose diagnosis/measurement reagent (kit) by using the technology of an enzyme colorimetric method and an enzyme coupling method, also relates to a method for measuring lactose concentration, and the composition and components of the reagent, and belongs to the technical field of medicine/food inspection and measurement. The reagent (kit) of the invention comprises the following components: buffer solution, coenzyme, phosphate group, lactase, glucose-6-phosphatase, glucose-6-phosphate dehydrogenase, and a stabilizing agent. A sample and a reagent are mixed in a certain volume ratio to undergo a series of enzymatic reaction, the reactant is placed under an ultraviolet/visible light analytical instrument, and the rising degree of the absorbance of the reactant at the main wavelength of 340nm is measured, so that the lactose concentration is measured.