Respiratory failure is the danger for the patients admitted in ICU, mechanical ventilation could save a lot of patients' life, however, it prolongs patients' ICU stay and hospital stay. High flow nasal cannula (HFNC) has advantages of PEEP effects, reducing dead space, increasing EELV and improving comfort, and it has been used to treat respiratory failure patients. In recent study, HFNC could improve mortality of respiratory failure patients with P/F<200mmHg in the subgroup. The investigators want to start a randomized controlled study to evaluate the effects of HFNC on the patients with respiratory failure.