Subarachnoid hemorrhage is one of the emergencies in brain surgery and one of the diseases with a high mortality rate; therefore, subarachnoid hemorrhage must be taken seriously. Subarachnoid hemorrhage is a type of hemorrhagic cerebrovascular disease and is divided into two types: primary and secondary. Primary subarachnoid hemorrhage is caused by the rupture and bleeding of blood vessels on the surface and base of the brain, and the blood flows directly into the subarachnoid space. It is also called spontaneous subarachnoid hemorrhage. Parenchymal or ventricular hemorrhage, traumatic subdural or epidural hemorrhage into the subarachnoid space is secondary subarachnoid hemorrhage. Subarachnoid hemorrhage is one of the most common neurological emergencies, accounting for 6-10% of acute cerebrovascular disease. The trigger of subarachnoid hemorrhage is the sudden onset of the disease in response to emotional stress, physical exertion, coughing, forceful defecation, alcohol consumption, and sexual intercourse. The main manifestations are sudden onset of severe headache, vomiting, impaired consciousness and neurological symptoms, a few may present with seizures and psychiatric symptoms, positive meningeal irritation signs on examination, etc. There are signs of hemorrhage on brain CT scan and uniform and consistent bloody cerebrospinal fluid on lumbar puncture. In summary, the diagnosis of subarachnoid hemorrhage is not difficult. The difficulty lies in finding the cause and choosing the curative treatment. The following possible causes of subarachnoid hemorrhage require the patience of a careful physician: 1) intracranial aneurysm, 2) hypertensive cerebral atherosclerosis, 3) arteriovenous malformation, 4) brain tumor, and 5) others: infected embolic aneurysm rupture, hematologic disease, periarteritis nodosa, systemic lupus erythematosus, and application of anticoagulants. By finding the cause, the next subarachnoid hemorrhage can be avoided.