Objectives To explore the long-term prognosis, treatment (especially dietary treatment) of classic maple syrup urine disease. Methods The complications and dietary treatment were observed by follow-up study of a classic MSUD patient. Results The patient have obvious damage in nervous system. However, reasonable dietary leucine tolerance therapy after the neonatal period can effectively reduce the metabolic crisis and complications. A mutation in BCKDHB gene was detected in the patient by genetic testing. Conclusion It is suggested that dietary restriction and monitoring of branched-chain amino acids are helpful to reduce the development of acute metabolic crisis and complications and improve the quality of life.