Hyperinsulinism-hyperammonemia (HI/HA) syndrome, often characterized by recurrent symptomatic hypoglycemia and persistent hyperammonemia, is the second most frequent cause of the congenital hyperinsulinism (CHI). Here, we reported a patient with normal birth weight, repeated seizures, untreatable hypoglycemia, and persistent, mild hyperammonemia. The genetic diagnosis revealed that the patient carried a heterozygous, de novo missense mutation (N410I, c.1401A>T) in the glutamate dehydrogenase 1 gene (GLUD1). The patient was treated with diazoxide, which significantly alleviated the hypoglycemia. CT and MRI brain scanning at different developmental stages revealed large-scale brain damage in the front lobe. Severe neurodevelopment deficits were identified in the follow-up.