Inclusion Criteria:  age≥18; Suspected lage vessel occlusion of acute anterior circulation occlusion; i.e. either the ICA or the M1/M2-segment of the MCA; Stroke symptom onset has been more than 6 hours but not more than 24 hours,and imaging confirmed the existenceof ischemic penumbra; NIHSS score≥6; Alberta Stroke Program Early CT score (ASPECTS) of 6-10 on non- contrast CT; (Level of consciousness) NIHSS score of 0 or 1 mRS score was 0-1 before stroke; Informed consent obtained;  Exclusion Criteria:  Rapid improvement in neurological status to an NIHSS < 6 or evidence of vessel recanalization prior to randomization; Seizures at stroke onset; Intracranial hemorrhage; Systolic pressure greater than 185 mm Hg or diastolic pressure greater than 110 mm Hg, or aggressive treatment intravenous medication)necessary to reduce blood pressure to these limits; Symptoms rapidly improving; Symptoms suggestive of subarachnoid hemorrhage, even if CT scan was normal; Platelet count of less than 100,000 per cubic millimeter; CT showed a multiple infarction (low density area greater than 1/3 cerebral hemisphere); severe hepatic or renal dysfunction; 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; inability to obtain informed consent; Life expectancy<90 days; Pregnant or breast-feeding women; Unwilling to be followed up or poor compliance for treatment; Patients being enrolled or having been enrolled in other clinical trial within 3 months prior to this clinical trial; Evidence of intracranial tumor; Baseline blood glucose of < 50 mg/dL (2.78 mmol) or > 400 mg/dL (22.20 mmol); Patients with upper gastrointestinal bleeding or nausea and vomiting who cannot use oxygen masks; Other circumstances requiring emergency oxygen inhalation;