The invention is about a method of measuring the content of blood ammonia, and it also concerns the reagent box of blood ammonia diagnosis. This invention belongs to the field of medical testing and measuring technology. The reagent box is consisted of buffer solution, 2ú¡ketoglutarate, reduced coenzyme, phosphoenolpyruvate, glutamate dehydrogenase, glutamate decarboxylase, phosphoenolpyruvate carboxylase, malate dehydrogenase and stabilizer. Firstly, we cause an enzyme-coupled reaction through mixing the sample and the reagent according to a certain proportion of volume; secondly, put the final reactant under the biochemical analyzer and test the absorbance variational situation (speed) of dominant wavelength; then we can get the content ofblood ammonia. By using this invention, we can get the necessary measuring result with high sensitiveness and fine precision through biochemical analyzer, and the result would not be contaminated by material of internal and exogenous sources. Thus, this method can be conveniently promoted and applied.