Arachnoid cysts may be caused by physiological reasons, such as congenital development, or pathologic reasons, such as hematoma of the brain, brain injury, meningitis, etc., and the cause should be identified in a timely manner. 1. Physiological causes: Arachnoid cysts are mostly caused by congenital development. It is mainly because the arachnoid membrane enters the subarachnoid space during the embryonic development period and grows and expands continuously, resulting in the appearance of arachnoid cysts. 2. Pathologic causes: (1) Brain hematoma, brain injury: when brain hematoma, brain injury, hematoma, cranial hypertension and so on can stimulate the arachnoid membrane, thus appearing the manifestation of arachnoid cyst. (2) Meningitis: after meningitis, the arachnoid membrane will be partially adhered, and the arachnoid membrane will be filled with cerebrospinal fluid or liquid with high protein content, and then arachnoid cysts will appear. Arachnoid cysts should be seen by a doctor in a timely manner to clarify the cause and, if necessary, treat the condition accordingly.