The invention provides a preparation method of a compound amino acid injection. The preparation method comprises the following steps: respectively washing a preparation container and a pipeline with alkaline water, a metal ion chelating agent solution and water for injection; putting the water for injection into a liquid preparation tank, adding sodium hydroxide, and stirring while adding until the sodium hydroxide is dissolved; introducing N2 into the liquid preparation tank, then sequentially adding tyrosine, isoleucine, leucine, lysine hydrochloride, methionine, threonine, valine, alanine,arginine hydrochloride, aspartic acid, glutamic acid, proline, serine, glycine, phenylalanine, histidine hydrochloride, xylitol and cysteine hydrochloride, and stirring and dissolving one by one; continuously introducing N2 into the liquid preparation tank, ensuring that the oxygen content is within 3%, cooling the materials, adding water for injection, adding tryptophan, stirring, dissolving, andsupplementing water to a full amount to obtain a mixed solution; and filtering and sterilizing the mixed solution, injecting the mixed solution into an infusion bottle, capping and sealing the bottle, and performing sterilizing and performing light inspection to obtain the compound amino acid injection.