Objective:To compare the effects of extrafine-particle versus fine-particle inhaled corticosteroids (ICS) combined with formoterol on clinical symptoms, airway inflammation and airway function in patients with bronchial asthma (referred to as asthma).Methods:This prospective, randomized controlled clinical trial enrolled a total of 111 patients diagnose of asthma and cough variant asthma with forced expired volume in one second (FEV 1) percentage of predicted (FEV 1%pred) >70% in Shanghai General Hospital, Shanghai Jiao Tong University School of Medicine from November 2020 to October 2022. The patients were divided into observation group (57 cases) and control group (54 cases) by random digits table method. The patients in observation group were treated with extrafine-particle ICS combined with formoterol, while the patients in control group were treated with fine-particle ICS combined with formoterol. Both groups were treated for 4 weeks. During treatment 10 patients lost follow up and a total of 101 patients complete the final study: 52 cases in observation group and 49 cases in control group. At baseline, the asthma control test (ACT) score was calculated to evaluate the clinical symptoms, fractional exhaled nitric oxide (FeNO) was applied to evaluate the airway inflammatory level, and the pulmonary function test and bronchodilation test were perfromed. The symptom relief time was record. After treatment, all of the parameters were reevaluated. Results:The FEV 1, peak expiratory flow (PEF), forced expired flow at 50% of forced vital capacity (MEF 50), forced expired flow at 25% of forced vital capacity (MEF 25), maximal expiratory flow-volume curve (MMEF) and ACT score after treatment in both groups were significantly improved compared to baseline, and there were statistical differences ( P<0.01). The improvement of ACT score in observation group was significantly higher than that in control group: (5.90 ± 2.25) scores vs. (4.10 ± 2.18) scores, and there was statistical difference ( P<0.01); there were no statistical differences in the symptom relief time and the improvement rates of FEV 1, PEF, MEF 50, MEF 25, MMEF between the two groups ( P>0.05). Further subgroup analysis was performed on 78 patients with small airway dysfunction, 39 patients were treated with extrafine-particle ICS combined with formoterol (observation subgroup), and 39 patients were treated with fine-particle ICS combined with formoterol (control subgroup). The improvement of ACT score in observation subgroup was significantly higher than that in control subgroup: (6.05 ± 2.22) scores vs. (3.95 ± 2.19) scores, and there was statistical difference ( P<0.01); there were no statistical differences in the symptom relief time and the improvement rates of FEV 1, PEF, MEF 50, MEF 25, MMEF between the two subgroups ( P>0.05). Spearman correlation analysis result showed that the improvement rates of MEF 50, MEF 25 and MMEF after treatment were positive correlated with baseline bronchodilator responses of FEV 1, FEV 1/FVC, MEF 50, MEF 25, MMEF ( r = 0.22 to 0.58, P<0.05), but not with baseline bronchodilator responses of PEF ( P>0.05). In terms of safety, neither of the two treatments had major adverse reactions that affect treatment. Conclusions:For asthma patients with baseline FEV 1%pred>70%, the extrafine-particle ICS combined with formoterol has more significantly improved of clinical symptoms compared to fine-particle ICS combined with formoterol, and potentially with better safety profile. The improvement ratio of small airway function parameters in baseline bronchodilation test could potentially predict treatment response.