Normoxia Hyperoxia (NBO) is a neuroprotective approach that can be implemented early. NBO is simple and non-invasive and can be used at home or in an ambulance to ensure the shortest possible time after cerebral ischemia occurs. The previous study by the investigators suggested that NBO therapy in the early stage of cerebral ischemia has a neuroprotective effect on ischemic brain injury. Although the neuroprotective effect of NBO has been demonstrated, the optimal duration of treatment for NBO to exert neuroprotective effect is still unclear. Therefore, further discussion of the duration of NBO treatment will contribute to the clinical application of NBO and provide a definite theoretical basis for the treatment of cerebral infarction.

Inclusion Criteria:

• Symptoms and signs were consistent with acute anterior circulation stroke,
• NIHSS score≥6分;Alberta Stroke Program Early CT score (ASPECTS)≥6;
• Met the indications for endovascular therapy;
• (Level of consciousness)NIHSS score 0 or 1; MRS score was 0-1 before stroke
• The time from onset to randomization was within 24 hours;
• Preoperative CTA or MRA confirmed the presence of large vessel occlusion (internal carotid artery or middle cerebral artery M1, M2 segments);
• Patients and their families signed informed consent

Exclusion Criteria:

• Rapid neurological function improvement, NIHSS score less than 10 points, or evidence of vessel recanalization prior to randomization;
• Seizures at stroke onset;
• Intracranial hemorrhage;
• Symptoms suggestive of subarachnoid hemorrhage, even if CT scan was normal;
• Known hemorrhagic diathesis, coagulation factor deficiency, or on anticoagulant therapy with INR > 3.0 or PTT > 3 times normal;
• Platelet count of less than 100,000 per cubic millimeter;
• Severe hepatic or renal dysfunction;
• Severe, sustained hypertension (Systolic Blood Pressure >185 mmHg or Diastolic Blood Pressure >110 mmHg)
• Baseline blood glucose of <50mg/dL (2.78 mmol) or >400mg/dL (22.20 mmol) Active and chronic obstructive pulmonary disease or acute respiratory distress syndrome;
• >3 L/min oxygen required to maintain peripheral arterial oxygen saturation (SaO2) 95% as per current stroke management guidelines;
• Medically unstable;
• Life expectancy<90 days;
• Patients who could not complete the 90-day follow-up;
• Evidence of intracranial tumor;
• Patients with anemia or polycythemia vera or other situations that require urgent oxygen inhalation;
• Patients with upper gastrointestinal bleeding or nausea or vomiting so that they cannot cooperate with the mask to inhale oxygen.
• A history of severe allergies to contrast agents;