High Flow Nasal Cannula(HFNC) oxygen therapy is a form of non –invasive respiratory support, which comprises an air oxygen blender, an active humidifier, a single heated circuit and a nasal cannula. It delivers adequately heated and humidified medical gas up to 60L/min, and is considered to have a number of physiological effects: reduction of anatomical dead space, PEEP effect, constant fraction of inspired oxygen and good humidification. It is widely used in RICU and ICU.