Objective ·To evaluate the abnormal rates of measured/predicted values of forced expiratory volume in 0.5 second (FEV_0.5) and FEV_0.75 in lung ventilation function testing, as well as the clinical significance of different cut-off points for improvement rates before and after bronchodilation testing in the diagnosis of children with bronchial asthma (asthma). Methods ·From January 2020 to February 2021, school-age children diagnosed with asthma during their hospitalization at the Children's Hospital, Zhejiang University School of Medicine were selected. The data on lung ventilation function testing and bronchodilator testing were collected. The difference in abnormal rates of measured/predicted values of FEV_t in children in lung ventilation function testing was compared. Diagnostic test four grid table was used to analyze the sensitivity and specificity of different cut-off points for the improvement rates of FEV_0.5 and FEV_0.75 in the diagnosis of asthma before and after bronchodilation testing. Results ·A total of 148 children were included. All enrolled cases completed lung ventilation function testing, with 51 cases completing bronchodilation testing simultaneously. The results of lung ventilation function testing showed that 21 patients had obstructive ventilation dysfunction, 30 patients had small airway dysfunction, and the remaining patients had normal, restrictive or mixed ventilation dysfunction. The results of bronchodilation testing showed that in the 51 asthmatic children with obstructive ventilation dysfunction and small airway dysfunction, 22 of them were positive for bronchodilation testing. The comparison of the abnormal rates of FEV_t measured/predicted values showed that the abnormal rates of FEV_0.75 measured/predicted values were higher than those of FEV₁ and FEV_0.5 (χ²=10.18, P=0.000; χ²=10.95, P=0.000). The sensitivity and specificity analysis results of different cut-off points for the improvement rates of EFV_0.75 and FEV_0.5 showed that the cut-off point for FEV_0.5 improvement rate at 16% had very high sensitivity and specificity, with the highest Jordan index; the cut-off points for FEV_0.75 improvement rate were 12% and 14%, respectively, with sensitivity and specificity reaching 100%. Conclusion ·The measured/predicted value of FEV_0.75 in lung ventilation function testing can be considered as an alternative indicator for the measured/predicted value of FEV₁ to further evaluate the presence of obstructive ventilation dysfunction. On the basis of the results of bronchodilation testing, it can be considered to analyze the airway reversibility by combining the best cut-off points of FEV_0.75 and FEV_0.5 improvement rates, in order to enhance the clinical value of bronchodilation testing in the diagnosis of asthma.