The surface of the human brain is composed of three layers of membranes: the soft meninges, the arachnoid membrane, and the dura mater, in order from the inside to the outside. The cavity between the arachnoid and the soft meninges is called the subarachnoid space, which is normally filled with colorless and transparent cerebrospinal fluid. When a cerebral blood vessel ruptures, blood flows into the subarachnoid space, which is called subarachnoid hemorrhage, or SAH for short. SAH is classified into traumatic and spontaneous according to the cause. The most common cause of spontaneous SAH is the rupture of intracranial aneurysm, which accounts for about 75%. Sclerosis, smog disease (Moyamoya disease), etc.; and about 10% of the causes are unknown. The clinical manifestations are sudden onset of severe headache, “tear-like” or “electric shock-like” headache, nausea, vomiting, neck straightening, severe convulsions, unconsciousness, and even respiratory and cardiac arrest, and about 10-15% of patients die before reaching the hospital. The cranial CT shows a high-density shadow in the subarachnoid space. CT may not be diagnostic when the amount of hemorrhage is small, but lumbar puncture of hemorrhagic hydrocephalus may confirm the diagnosis. Once subarachnoid hemorrhage is clearly identified, the cause of SAH must be clarified, and non-invasive diagnostic tools such as MRA (magnetic resonance angiography) and CTA (tomographic angiography) can be used as screening, but the gold standard for confirming the cause is still invasive digital subtraction cerebral angiography, or DSA, which must be done as early as possible to clarify the cause. If the aneurysm is an aneurysm, aim to treat the aneurysm as early as possible (within 3 days of the acute phase). This includes craniotomy and interventional embolization, and the exact type of procedure requires evidence-based medicine and physician-patient communication. If the SAH is caused by a ruptured cerebrovascular malformation, elective surgery, including craniotomy and interventional embolization and gamma knife treatment, can be performed after the hemorrhage is absorbed and the condition is stabilized. If SAH is caused by smog, surgical treatment such as temporal muscle patching and vascular bypass can be considered at an elective stage. In some cases, the cause of SAH cannot be found, i.e., the cerebral angiography result is normal, and the DSA will be reviewed after the hemorrhage is absorbed to avoid missed diagnosis.