Chronic obstructive pulmonary disease is graded according to the percentage of first second exertional expiratory volume of lung function to the expected value, and is classified into four grades, mild, moderate, severe, and very severe. First second expiratory volume with force >80% of the expected value is mild; first second expiratory volume with force 50%-80% of the expected value is intermediate; first second expiratory volume with force 30%-50% of the expected value is severe; and first second expiratory volume with force less than 30% of the expected value is very severe. There is also a condition with chronic respiratory failure, where the first second exertional expiratory volume is less than 50% of the expected value, which can also be classified as very severe. The above grading is used to determine the mild, moderate, severe and very severe degrees of slow obstructive pulmonary disease, and the corresponding medication selection is made according to the different levels of classification. The different classifications can also determine the prognosis of patients with chronic obstructive pulmonary disease, and can serve as a reminder for future medication use.