Lung function test is a physical examination method that allows patients to “blow” according to the instructions, and the instrument gives the analysis results without any damage to the body, pain and discomfort. Compared with X-ray chest X-ray, CT and other tests, lung function test focuses more on understanding the functional changes in the lungs, and is an important means of examination for respiratory diseases (e.g. bronchial asthma, chronic obstructive pulmonary disease, cough variant asthma, hyperventilation syndrome, etc.). Lung function test can understand the physiological state of the respiratory system, clarify the mechanism and type of lung dysfunction, determine the degree of lesion damage, estimate the functional reserve of the lungs, and provide reference for doctors’ diagnosis and treatment. Lung function tests have the advantages of high sensitivity, convenient repeat testing and easy acceptance by patients. Lung function tests are helpful in confirming the diagnosis of asthma and are also an important basis for assessing the degree of asthma control. In patients with asthma symptoms but normal lung function, measurement of airway responsiveness and intraday variability of PEF can help confirm the diagnosis of asthma.