Arginine accounts for about 6% in human hair, and there is almost no loss of arginine in the mother liquor after extracting cystine. Therefore, after the mother liquor is desalted, the arginine is separated by exchange with a cation exchange resin. According to the exchange capacity of the resin to arginine, as long as the amount of the resin is controlled, the arginine can be exchanged and exchanged without being exchanged with other amino acids. Then, the arginine is eluted with an alkaline eluent, and the eluate is concentrated under reduced pressure to obtain crystal arginine.