Pulmonary function is a test to assess pulmonary ventilation and air exchange function. It is mainly used in interstitial pneumonia for diagnosis, assessing the severity of the disease, evaluating the effectiveness of treatment, and speculating on prognosis. In patients with interstitial pneumonia, the findings of pulmonary function are usually restrictive ventilation dysfunction and reduced diffusion function, and some patients may exhibit mixed ventilation dysfunction, mainly seen in smokers. The severity of the patient is assessed based on the percentage of exertional spirometry as a percentage of the expected value and the percentage of decreased diffusion function, which usually coincides with the patient’s symptoms as well as imaging, but some patients have mild imaging but heavy symptom and pulmonary function manifestations, when the presence of comorbidities is often indicated and attention is paid to detection and differentiation. The degree of improvement in lung function is one way to evaluate the effectiveness of a treatment regimen. In specific patients with pulmonary fibrosis, there is no way to evaluate whether a particular treatment is effective in them because the decline in lung function in these patients is not currently reversible, meaning that the symptoms as well as lung function in these patients will still worsen and decline over time. Pulmonary function is judged by the results of existing large clinical trials: Patients with pulmonary fibrosis usually lose about 130-210 ml of lung capacity a year, and a treatment can be said to be effective if it can significantly slow down the rate of decline in lung function.