Objective To detect the levels of neutronphil elastase (NE) and alpha1 antitrypsin (α1-AT) in serum of chronic obstructive pulmonary disease (COPD) group and health control group, and analyzed the relationship between the levels and pulmonary function indicator or quantity of smoking.Methods Fifty six cases of serum samples with COPD, and 21 cases of serum samples from health control were collected and analyzed. The COPD group was divided into two groups, one of acute exacerbation phase (AE-COPD), and the other of stable phase (ST-COPD). The levels of serum NE and α1-AT in aboved groups were measured by enzyme-linked immunoadsordent assay (ELISA) method.Results ①The level of NE in AE-COPD group is significantly higher than which in health control group ( t =3.91, P ＜0.01). The level of NE in ST-COPD group is significantly higher than which in health control group(( t =2.02, P ＜0.05). There are no significant difference between the level in AE-COPD group with the level in ST-COPD group ( t =1.91, P ＞0.05). The level of α1-AT in AE-COPD group are significantly worse than which in health control group( t =-4.45, P ＜0.01 ). There are significant difference between the level in ST-COPD with the level in health group( t =-2.73, P ＜0. 05), and with the level in AE-COPD group( t =-3.19, P ＜0.01 ). ②There is a negative correlation between the levels of NE with α1-AT in AE-COPD group ( r =-0.71, P ＜0.01). There is no correlation between the levels of NE with α1-AT in ST-COPD group( r =0. 33, P ＞0.05). ③There is a negative correlatin between the levels of NE with FEV1 ％ pred in AE-COPD group( r =-0.82, P ＜0.01). There are positive correlation between the levels of NE with RV/TLC or the amount of smoking in AE-COPD ( r =0.74,0.73, P ＜0.01). In ST-COPD group, there are no correlation between the level of NE with FEV1 ％ pred or RV/TLC ( r =0.25,-0.23, P ＞0.05), and a positive correlation between the level and the amount of smoking ( r =0.42, P ＜0.05). ④In AE-COPD group, there are a positive correlation between the level of α1-AT with FEV1 ％ pred ( r =0.86, P ＜0.01), and negative correlation between the level with RV/TLC or smoking ( r =-0.83,-0.80, P ＜0.01). In ST-COPD group, there are a positive correlation between the level of α1-AT with FEV1 ％pred ( r =0.47, P ＜0.05), and a negative correlation between the level with RV/TLC (r =-0.46, P ＜0.05), and no correlation between the level with smoking. Conclusions① The imbalance of NE and α1-AT plays an important role in the pathogenesis of COPD. ② The incremental level of NE and the content relative shortage of α1-AT are correlated with airway inflammation and airway remodeling, and important factors in the process of forming emphysema. ③Smoking can lead to incremental level of NE and content relative shortage of α1-AT, and which is one of the risk factors of COPD.