Chronic obstructive pulmonary disease(COPD) is a preventable and treatable illness, which is clinically characterized by persistent airflow limitation that is usually progressive. It's well known that airflow limitation lead to air trapping particularly during exercise, resulting in increased dyspnea and limitation of exercise capacity. Thus, COPD patients often unconsciously adopt a sedentary lifestyle, probably due to distressing exertional dyspnea. The sedentarism results in physical deconditioning and alterations of the peripheral muscles, and the latter one causes more exertional dyspnea and fatigue, which in turn leads to much more sedentarism, forming the vicious cycle of sedentarism, dyspnea/fatigue and more sedentarism. Thereby, measures should be taken to interrupt this vicious cycle. Some previous studies suggested that niv acting as an add-on therapy could help unloading the inspiratory muscle, reducing the dynamic compression of the airway, diminishing the dynamic hyperinflation,to some extent, relieved dyspnea during exercise in COPD patients. However,those measures were taken during exercise, acting as an add-on therapy. Whether NIV can play a role as a rescue therapy to relieve exertional dyspnea while the patients are already undergoing optimal treatment with bronchodilator and anti-inflammatory medication is still not quite clear. Therefore, we hypothesize that niv as a rescue-therapy has the same effect on relieving dyspnea in stable severe COPD patients just like the add-on therapy.