The invention relates to a method for determining the activity of adenosine deaminase, and also the reagent kit for adenosine deaminase diagnosis. The reagent kit comprises cushioning solution, adenosine, glutacid, deacidized type coenzyme, adenosine triphosphate, pyroracemic acid, phosphoenolpyruvate phosphatase, glutamine synthetase, pyruvic oxidase, phosphoenolpyruvate pyruvate carboxylase, malic dehydrogenase, and stabilizer. By mixing sample and reagent of a predetermiend volumetric ratio, generating coupling reaction between them, subjecting the final reactant to biochemiscal analyser, the main wavelength absorbancy variance ratio (speed) can be detected, and the activity of the adenosine deaminase can thus be measured. The method of the invention can be used to obtain the needed measurement result purely through biochemical analytic instruments, and advantages of the method include higher sensibility, better accuracy, less susceptibility to contamination of internal or external materials, and easy application.