The understanding of spontaneous subarachnoid hemorrhage as well as intracranial aneurysm has gone through a long process from etiology, pathogenesis, diagnosis and treatment, and finally we defined subarachnoid hemorrhage (SAH) as the entry of blood into the subarachnoid space and brain pool after cerebral vascular rupture, which is divided into traumatic and non-traumatic (i.e., spontaneous SAH). The main cause of spontaneous SAH is intracranial aneurysm, and other causes include cerebrovascular malformation, smog, non-aneurysmal hemorrhage around the midbrain, dural arteriovenous fistula, cerebral atherosclerosis and hypertension, hematologic disease, allergic disease, allergic violet scar, infection, poisoning, and tumor. And what is intracranial aneurysm? It is a cerebral vascular aneurysm-like protrusion due to abnormal changes of local blood vessels. It can be divided into four categories according to the size of diameter: less than 0.5cm for small aneurysm, 0.5cm~1.5cm for general aneurysm, 1.5cm~2.5cm for large aneurysm, and more than 2.5cm for giant aneurysm. Intracranial aneurysms account for 80% of spontaneous subarachnoid hemorrhage, and the most common cause is still congenital. Most aneurysms are saccular in shape and occur at the arterial bifurcation of the arterial ring at the base of the brain, which is associated with the weakest middle layer of the artery and is subject to the greatest blood flow impact. Acquired factors are mainly related to atherosclerosis and are called atherosclerotic aneurysms. This is followed by infected aneurysms (also known as mycotic or bacterial aneurysms) and traumatic aneurysms (also known as pseudoaneurysms). Other aneurysms can be associated with a small number of anomalous vascular network disorders of the skull base, cerebral arteriovenous malformations, intracranial vascular developmental anomalies, and cerebral artery occlusions. About 3/4 of patients have a headache, which is a sudden, atypical headache of varying severity that has not been experienced before; more than 50% have transient disturbance of consciousness; a minority of patients have headache, dizziness, vision changes, or neck stiffness within two weeks prior to onset; and some patients may have nausea and vomiting. Physical examination may include signs of meningeal irritation, pyramidal fasciculus (hemiparesis), fundus hemorrhage (Terson’s sign), and focal signs: the most common sign is actinic nerve palsy. When the above conditions appear, you should go to the hospital in time to get a clear diagnosis and early treatment to prevent delays.