 For AECOPD patients, only 8 patients ventilated by noninvasive and invasive positive pressure ventilation. Nasal cannula is the most common pattern of oxygen therapy in mild AECOPD. As a low flow oxygen therapy, nasal cannula has many disadvantages.Therefore, we design a randomized controlled trialRCTto explore whether HFNC would be better than nasal cannula to prevent the aggravation of respiratory failure and endotracheal intubation in mild AECOPD. 