 Patients with COPD are divided into two groups: moderate group and severe group, both of which are intervented by CPAP level. The static pressure volume curves of the lung and chest wall Campbell diagram were established with slow deep respiratory manoeuvre. Before using CPAP level, we will measure the relevant parameters of lung volume, respiratory flow, diaphragm electromyogram, central drive mechanical and ventilation coupling. Then incremental CPAP level will be applied to investigate the effects of CPAP level on the above mentioned respiratory mechanics parameters. 