Background Understanding asthmatic airway structural changes and the bronchodilator responses may help unravel targets for intervention. However, structural abnormalities of asthmatic airways with different disease severity and the major anatomical site of bronchodilator responses have not been well elucidated. We aim to evaluate the airway remodeling characteristics and the bronchodilator responses in medium-sized and small airways of asthma. Methods We recruited 104 asthmatic patients and 31 non-smoking control subjects to compare the airway inner area (Ai) and airway wall area percentage (Aw%) with endobronchial optical coherence tomography. We also enrolled 32 patients with moderate-to-severe asthma to dynamically assess the airway morphological changes after salbutamol inhalation. Results More prominent airway structural abnormalities correlated with greater asthma severity, evidenced by the decreased Ai and greater Aw% in medium-sized and small airways. Patients with mild asthma yielded comparable Ai but greater Aw% than control subjects. Salbutamol inhalation led to a rapid dilatation of both medium-sized and small airways, the lung function improvement correlated significantly with the increase in Ai of the medium-sized, but not small, airways at 15 min. Conclusion Luminal narrowing and airway wall thickening of the medium-sized and small airways are present in mild asthma and reflect asthma severity, lending support to the use of anti-imflammatory intervention in mild asthma. The medium-sized airways are the crucial site of the bronchodilator responses, providing the scientific rationale for future development of more effective delivery of inhaled medications for asthma.