The invention relates to a kit for diagnosing/measuring lithium by utilizing the technologies of the enzymic colorimetry and the enzyme linked immunosorbent assay, wherein, the activity of the kit can be inhibited by lithium. The invention further relates to a method, a principle and the composition and the components of a reagent for measuring the concentration of lithium, and belongs to the technical field of medical/industrial/environmental inspection and measurement. The main components of the kit include a buffer solution, reduced coenzyme, magnesium chloride, inositol-1-phosphate, an oxaloacetic acid, a pyruvic acid, inositol-1-phosphatase, phosphoenolpyruvate carboxykinase, malic dehydrogenase and a stabilizer. Through mixing a check sample and a lithium sample respectively with the reagent by a certain volume ratio, a series of enzymatic reactions occur, then the reactant is placed under an ultraviolet/visible light analyzer, the degree/velocity of the decrease in absorbance at 340 nm of the dominant wavelength is detected, and the difference in the degree/velocity of the decrease between the check sample and lithium sample is compared, thereby measuring the concentration of lithium.