The most common causes of subarachnoid hemorrhage are congenital intracranial aneurysms and vascular malformations, followed by hypertensive cerebral atherosclerosis, intracranial tumors, hematologic diseases, arteritis caused by various infections, tumors that destroy blood vessels, and abnormal vascular network disease of the skull base (moyamoya disease). There are also unexplained subarachnoid hemorrhages, which are those for which no cause is found by whole brain angiography and brain CT scan. Among the causes, cornu aneurysms account for about 75%, with an annual incidence of 6/100,000; arteriovenous malformations account for about 10%, mostly in young people, and more than 90% are located on the curtain; anomalous vascular network at the base of the brain (Moyamoya disease) accounts for 20% of SAH in children; and unknown causes account for 10%. It is generally believed that the onset of SAH before the age of 30 is mostly due to vascular malformation, after the age of 40 is mostly due to intracranial aneurysm rupture, and above the age of 50 is often caused by hypertensive cerebral atherosclerosis and brain tumor. In addition, smoking and alcohol consumption are closely related to subarachnoid hemorrhage. Foreign animal experiments have demonstrated that there are 3 factors in the formation of aneurysms: Willis ring pressure, hypertension and increased vascular fragility. Smoking affects these 3 factors, causing an acute increase in blood pressure that gradually decreases after 3 h. This is consistent with the fact that the highest incidence of subarachnoid hemorrhage seen clinically is within 3 h of smoking. Smoking also activates pulmonary macrophage activity and promotes the release of hydrolases, which can cause lung damage and increased cerebrovascular fragility, thereby increasing the risk rate of subarachnoid hemorrhage. Heavy alcohol consumption may cause hypertension or other alterations including blood clotting mechanisms and cerebral blood flow, which may accelerate the development of subarachnoid hemorrhage. The incidence of subarachnoid hemorrhage has been reported to be twice as high in excessive drinkers as in non-drinkers.