Objective To better understand the oxygen uptake kinetics during the 3 minutes of unloaded exercise of cardiopulmonary exercise test (CPET) in patients with severe chronic obstructive pulmonary disease(COPD), to investigate the relationship between the mean response time (MRT)for oxygen consumption(VO2)and the exercise capacity, and to evaluate the usage of the MRT as one of the parameters to judge the cardiopulmonary functions of the patients with severe COPD. Methods We evaluated 41 severe patients with COPD and 30 normal peoples, who had the pulmonary function test (PFT)and cycle ergometer CPET studies to maximum tolerance. Gas exchange were calculated breath by breath. Results No adverse events occurred during CPET. MRT for VO2 were consistently found correlated well with peak VO2,peak VO2 %pred, VO2/kg and GOLD class ( P ＜0.001). And also the parameters from pulmonary function test, such as FVC,FEV1 ,FEV1 %pred,FEV1/FVC, MVV,MVV% pred,DLco 及 DLco %pred( P ＜0. 001). Conclusions Patients with several COPD can safely undergo the cycle ergometer CPET to their maximal tolerance. The MRT for VO2 in the unload exercise of CPET can be used to evaluate the cardiopulmonary function of the severe patients with COPD.