External hydrocephalus (EH) is a specific form of traffic hydrocephalus in infancy and is a benign, self-healing condition. In the past, due to the lack of awareness of this disease, it was often confused with subdural effusion or cerebral atrophy, which easily led to misdiagnosis. With the widespread use of CT, the understanding of this disease has gradually improved. EH is a manifestation of abnormal brain development in infants caused by various etiologies, and is most common in infants aged 2 to 24 months with unclosed fontanelles, with a male predominance. The clinical picture is one of enlarged head circumference, bulging fontanelle and seizures, but the infant’s development and intelligence are normal. The pathogenesis of EH is still unknown. It is generally believed that EH only occurs in infants with unclosed fontanelles and unclosed cranial sutures, and that open cranial sutures are necessary for EH to occur. When the cranial suture is open, the arachnoid granules become dysfunctional in absorbing cerebrospinal fluid, which is the main factor leading to subarachnoid fluid accumulation. Secondary EH is caused by certain factors, such as neonatal hypoxic-ischemic encephalopathy, encephalitis, meningitis, traumatic brain injury, premature birth, etc., which cause mechanical or inflammatory changes in the arachnoid membrane, thus affecting the absorption of cerebrospinal fluid by the arachnoid granules, while inflammatory changes and hypoxia stimulate increased secretion of the choroid plexus, all of which cause imbalance in the secretion and absorption of cerebrospinal fluid. These changes cause imbalance in cerebrospinal fluid secretion and absorption, resulting in an increase in circulating cerebrospinal fluid and the formation of temporary hydrocephalus. Hydrocephalus can cause intracranial hypertension, but in infancy, because the fontanelle and cranial sutures are not yet closed, this increased pressure is buffered by the cranial sutures and the bulging fontanelle, making the symptoms less obvious. Primary EH is a kind of benign temporary hydrocephalus, if there is no obvious symptom, it can be reviewed regularly and can disappear on its own without treatment, and there is no sequelae. Secondary EH should vary depending on the etiology and clinical presentation. Mild cases and asymptomatic or occasional cases can be treated without treatment, and most children can improve on their own after removing the cause. More severe cases should be treated aggressively, otherwise the development of the infant will be affected.