The severity of chronic obstructive pulmonary disease is graded into four levels, I, II, III and IV. I is mild, II is moderate, III is severe and IV is very severe. It mainly relies on three elements to determine: 1. The ratio of the first second forceful expiratory volume FEV1 and forceful spirometry FVC is to be less than 70% in all four grades; 2. Based on the percentage of the first second forceful expiratory volume in the expected value. Mild should be greater than 80%, grade II is 50%-80%, grade III is 30%-50%, and grade IV is less than 30%; 3. According to clinical symptoms, clinical symptoms are different for different grades. grade I can have no obvious symptoms, or mild symptoms, such as chronic cough and cough. Grade II would have obvious symptoms of chronic cough and coughing sputum. In severe grade, in addition to chronic cough and sputum, dyspnea may also occur. When the patient reaches grade IV, the dyspnea is very severe and hypoxemia is detected before the patient can be considered as grade IV, which is the most severe.