Small airways are airways with a diameter of 3 mm or less, and most of them are fine bronchi, the latter can be divided into membranous fine bronchi and respiratory fine bronchi. The diameter of fine bronchi is 0.5~1mm. Because of their large number and large total cross-sectional area, they do not easily form resistance to airflow. Therefore, by the time symptoms and lung function changes occur, a significant number of small airways have already been damaged. Small airway lesions can originate from themselves or can spread from larger bronchial and parenchymal lung lesions. Morphologic changes in small airway abnormalities include wall thickening due to muscle thickening, inflammation and fibrosis, narrowing or occlusion of the fine bronchi, dilatation of the fine bronchi and mucus embolism, which can occur in a variety of diseases and vary in severity. In CT examinations of cases with suspected small airway lesions, HRCT images with thin layers less than 1.25 mm are obtained; direct signs of HRCT include thickening of lobular central structures, dendritic signs and dilated fine bronchi, and indirect signs include mosaic signs of air trapping due to small airway obstruction.