Phenylketonuria is one of the few treatable genetic metabolic diseases. It strives for early diagnosis and treatment to avoid irreversible damage to the nervous system. Once diagnosed, aggressive treatment should be given. The younger the age at which treatment starts, the better the effect. The main approach is to use a low-phenylalanine formula milk. When the blood concentration drops to a normal level, natural diet can gradually be added in small amounts, with breast milk being the preferred choice, as it contains only one-third the phenylalanine of cow's milk. Larger infants can add cow's milk, porridge, noodles, eggs, etc. The added foods should also follow the principle of being low in protein and low in phenylalanine, because concentrations of phenylalanine that are too high or too low will affect growth and development. Low-phenylalanine treatment should continue at least until after puberty. Lifelong treatment is more beneficial for patients. This disease can only control symptoms, and lifelong treatment is the best; it cannot be completely cured.