The invention relates to a homocysteine diagnosing / measuring reagent (kit) using the technologies of an enzyme colorimetric method and an enzyme linkage method. At the same time, the invention also relates to a homocysteine concentration measuring method, the reagent composition and reagent ingredients, which belong to the technical field of medical detection and measurement. The reagent (kit) mainly comprises the following ingredients: a buffer solution, cozymase, sodium bicarbonate (carbon dioxide), adenyl pyrophosphate, glyceraldehyde-3-phosphate, homocysteine desulfhydrase, carbamyl phosphate synthetase, glyceraldehyde-3-phosphate dehydrogenase and stabilizing agents. The method has the following steps: mixing samples and the reagent by the certain volume percent for making the samples and the reagent take a series of enzymatic reaction, and then, placing reactants under an ultraviolet / visible light analyzer to detect the rising degree of the light absorbancy in the position with the main wave length of 340 nm, so the concentration of the homocysteine can be measured and calculated.