Objective: To explore the changing pattern of small airway function and its dilation test in patients with chronic cough and cough variant asthma, and to explore the possibility of incorporating small airway dilation test variability into the diagnostic criteria of CVA. Methods: All included patients underwent forced vital capacity test and bronchial dilation test(BDT), and the diagnosis of CVA was confirmed by a positive dilation test according to the FEV1 criteria, and was regarded as the dilation test positive group. Bronchial provocation test(BPT) was performed 3 days later in patients with negative bronchial dilation test. A positive diagnosis of CVA was made and was regarded as the positive provocation test group. The two groups were all labeled as the CVA group. The changes in the function of small airways of all patients were analyzed to explore the pattern of change and its diagnostic value for CVA. Results: A total of 66 patients with suspected CVA were included, 58 cases were diagnosed as CVA, including 14 cases with positive bronchial dilation test and 44 cases with positive BPT. There were 36 cases in the normal control group. There were no differences(P> 0.05) in gender, age, height, weight, the proportion of smokers between the CVA group and the normal control group. Lung function of the two groups: FVC(L): 3.50±1.55 in the CVA group, 3.57±0.76 in the control group, there was no difference between the two groups(P>0.05). FEV1(L·s^-1): 2.86±0.86 in the CVA group, 2.97±0.60 in the control group, there was a difference between the two groups(P<0.05). FEF75%(L·s^-1): 2.47±1.80 in the CVA group, 2.58±1.51 in the control group, there was a difference between the two groups(P<0.01). FEF50%(L·s^-1): 2.33±1.80 in the CVA group, 2.41±1.18 in the control group, there was a difference between the two groups(P<0.01). There was a difference of FEF25%(L·s^-1) between the two groups(P>0.05): 1.15±0.53 in the CVA group, 1.53±0.45 in the control group. The average variation rates(%) of FEV1, FEF75%, FEF50%, and FEF25% in the CVA group were: 8.7±7.2, 14.4±13.5, 17.3±15.2, and 21.6±21.1, respectively, and the first three did not differ from each other(P＞0.05), while the variation rate of FEF25% was significantly higher than that of the three. There were 12 positive FEV1 dilation tests, with a positive rate of 20.7%(12/58), 13 positive FEF75% dilation tests, with a positive rate of 22.4%(13/58), 11 positive FEF50% dilation tests, with a positive rate of 19.0%(11/58), and 11 positive FEF75% dilation tests, with a positive rate of 19.0%(11/58), and FEF25%-75% dilation test was positive in 15 cases with a positive rate of 25.9%(15/58). Taking FEF25% variation rate greater than 25% as the criterion for diagnosing CVA, the positive rate of its diagnosis was 41.4%(24/58), which was significantly higher than the other indicators. Conclusion: Small airway function changes are more obvious in patients with CVA, and small airway dilation test has a higher diagnostic rate for CVA, and is used for the diagnosis of clinical CVA, which is simple, convenient, safe to be applied clinically.