Over half of chronic obstructive pulmonary disease (COPD) patients develop pulmonary hypertension. The current therapy focuses only on the basic disease and there are a lot of controversies about the use of PAH target therapy in group 3 pulmonary hypertension. Our study is to explore whether sildenafil, a pulmonary arterial hypertension (PAH) target drug, could be efficient and safe in improving symptoms and survival of severe pulmonary hypertension caused by COPD.