The invention discloses compound amino acid injecta, which contains arginine hydrochloride, histidine monohydrochloride, leucine, isoleucine, lysine hydrochloride and the like; meanwhile, the invention further discloses a preparation method of the compound amino acid injecta (18AA); in the event of ensuring the product quality, the compound amino acid injecta has the characteristics of saving energy and enhancing efficacy; by means of repetitive verification on precision, detection limit, quantitation limit, linear range and the like, a simple, convenient, rapid and reliable method with cleaning verification is ensured; furthermore, the residual quantity of antioxygen, namely sodium hydrogensulfite, in the compound amino acid injecta and sulphate ion generated by degrading sodium hydrogensulfite are detected according to an ion chromatography principle, thus, the amount of the antioxygen charged in the raw material proportioning can be accurately measured, and the stability of the raw material proportioning process is judged according to the measurement result; and a detection method of the compound amino acid injecta provided by the invention is an online derivative amino acid content measuring method, the speed is rapid, and the method is applied to large-scale detection.