For stable COPD:  Inclusion Criteria:  Over 40 years old. in line with the diagnostic criteria in GOLD 2018. with the ability to sign informed consent. can be followed up according to the study protocol.  Exclusion Criteria:  with other pulmonary diseases (such as lung cancer, sarcoidosis, tuberculosis, pulmonary fibrosis, cystic fibrosis) and severe α1-antitrypsin deficiency. with other inflammatory diseases (such as rheumatoid arthritis and inflammatory bowel disease, etc.). a history of pulmonary surgery, or a diagnosis of malignant tumor recently. a history of blood transfusion in 4 weeks. participating in a double-blind drug clinical trial. unable to walk. using oral or intravenous steroid therapy. with acute exacerbation in 4 weeks.  For AECOPD:  Inclusion Criteria:  Patients who were over 40 years old were eligible for inclusion if they had a diagnosis of COPD in their primary care clinical record and were presenting with an acute exacerbation of respiratory symptoms.  Exclusion Criteria:  Other specific diseases that may account for sudden changes in respiratory symptoms should be excluded by clinical or laboratory tests. The remaining exclusion criteria were consistent with the above five criteria in stable COPD.  The control group:  Inclusion Criteria:  Healthy people over 40 years old.  Exclusion Criteria:  with chronic airway diseases and other lung diseases, and heart, kidney, liver, and other important organ diseases.