The greater occipital pool is an anatomical structure in the brain that is located below the cerebellum, in the lower posterior part of the posterior fossa of the skull, and upwardly can be connected to the fourth ventricle. Enlarged occipital pool means that the size of the occipital pool is enlarged compared to normal. The presence or absence of occipital pool, how it is shaped and the size of the pool varies from person to person, and the shape and size, in particular, are often variable. Generally an enlarged occipital pool can be seen at any age, it is more of an imaging term than a disease, and in most cases is due to an arachnoid cyst. The patient usually has a normal fourth ventricle volume and morphology, with no obvious occupying effect or hydrocephalus, etc., and does not need to be treated for this, but can be reviewed and followed up with a cranial MRI once every six months, and if there is a continued enlargement of the occipital pool or if it produces a compressive effect, surgery is considered.