Abstract: Objective To compare the lung function and clinical features of asthma-COPD overlap syndrome(ACOS), asthma, and COPD for improving the early diagnosis and treatment of ACOS. Methods Selected patients in department of respiratory Chinese PLA General Hospital were classified as ACOS group(n=30), asthma group(n=30) and COPD group(n=30).Clinical data were collected and compared between three groups, including gender, age, pulmonary function, fractional exhaled nitric oxide(FeNO), peripheral eosinophil count, total level of IgE and clinical symptom scores. Results The mean age of patients in ACOS group was(59.1±12.0) years with 50% of patients over the age of 60.The predicted level of forced expiratory volume in 1 second(FEV₁%) was significantly lower in ACOS group than asthma group(55.01%±21.26%) vs(71.52%±24.80%), P< 0.05; Before inhalation of short-actingβ₂-agonists, the FEV₁/FVC% was significantly lower in ACOS group than asthma group(51.11%±13.17%) vs(63.95%±13.54%), P< 0.05; The number of patients with small airway function obstacle in ACOS group was significantly greater than that in asthma group29 cases(96.7%) vs 23 cases(76.7%), P< 0.05; And RV/TLC% in ACOS group was significantly higher than asthma group(46.9%±10.5%) vs(30.3%±6.5%), P< 0.05.DLco-SB% in ACOS group was significantly lower than asthma group(60.9%±15.9%) vs(88.3%±16.6%), P< 0.05, but it was significantly higher than COPD group(60.9%±15.9%) vs(50.5%±16.0%), P< 0.05.While, there was no statistically significant difference in FEV₁% predicted, FEV₁/FVC%, proportion of patients with small airway function obstacle and RV/TLC% between ACOS group and COPD group.The peripheral eosinophil count in ACOS group was significantly lower than asthma group(4.12%±3.86%) vs(6.65%±6.17%), P< 0.05.The proportion of patients with increased total IgE accounted for 60% in ACOS group, 56.7% in asthma group and 10% in COPD group.FeNO value in ACOS group was significantly lower than asthma group(43.75±24.29) ppb vs(63.90±52.97) ppb, P< 0.05, but it was significantly higher than COPD group(60.9%±15.9%) vs (50.5%±16.0%), P< 0.05. While, there was no statistically significant difference in FEV1% predicted, FEV1/FVC%, proportion of patients with small airway function obstacle and RV/TLC% between ACOS group and COPD group. The peripheral eosinophil count in ACOS group was significantly lower than asthma group(4.12%±3.86%) vs (6.65%±6.17%), P< 0.05. The proportion of patients with increased total IgE accounted for 60% in ACOS group, 56.7% in asthma group and 10% in COPD group. FeNO value in ACOS group was significantly lower than asthma group(43.75±24.29) ppb vs (63.90±52.97) ppb, P< 0.05, but it wa higher than COPD group(43.75±24.29) ppb vs (32.53±18.33) ppb, P< 0.05. There was no statistically significant difference in clinical symptom scores between three groups. Conclusion Compared with asthma group, patients in ACOS group are older with more significant decrease in pulmonary ventilation function, higher proportion of patients with small airway function disorder, higher residual capacity, lower diffusion capacity, lower FeNO value and peripheral eosinophil count. However, patients in ACOS group show higher diffusion capacity and higher proportion of elevated total IgE when compared with patients in COPD group.