Abstract： Objective:To determine the correlation between change of respiratory impedance and small airway lesions in chronic obstructive pulmonary disease (COPD) patients with different eosinophilic granulocyte levels by impulse oscillometry system (IOS).Methods:This was a case-control study.Random sampling method was used to select patients with COPD at stable stage at the time of outpatient treatment in Shanghai Pulmonary Hospital from December 2020 to December 2021, and the patients were assigned into two groups according to the peripheral blood eosinophil ratio during screening.Patients with eosinophil ratio ≥2% were included in Group A, and patients with eosinophil ratio <2% were included in Group B. A total of 106 patients with COPD were included, 94 patients were involved in the present research finally, and proportion of male patients was 74%.Relevant clinical data were collected at the time of enrollment.Routine pulmonary function indexes and IOS indexes were measured, and then small airway lesions and respiratory impedance were analyzed.Results:Viewed from the enrollment group, the proportion of patients with continued ascent of peripheral blood eosinophilia was 50%, from the smoking index, Group B were higher than that of Group A ( P<0.05).Differences in respiratory impedance at the percent of predicted value (Z5%pred), resonant frequency (Fres), respiratory resistance in percent of predicted value (R5%pred), peripheral airway resistance (R5-R20) and peripheral airway resistance (X5) between two groups were statistically significant (all P<0.05).The results of Pearson′s study showed that X5 with the percentage of forced expiratory volume in the first second of prediction value (FEV 1%pred), FEV 3%pred, percentage of flow rate at 25% vital capacity of exhalation force (FEF 25%pred), FEF 50%pred and FEF 75%pred were positively correlated ( r values were 0.57, 0.49, 0.56, 0.54, and 0.40, respectively, all P<0.05), Z5%pred, R5%pred and Fres were compared with FEV 1%pred ( r values were -0.57, -0.43, and -0.89, all P<0.05), FEV 3%pred ( r values were -0.45, -0.30, and -0.73, respectively, all P<0.05), FEF 75%pred ( r values were -0.57, -0.44, and -0.76, respectively, all P<0.05), FEF 50%pred ( r values were -0.55, -0.44, and -0.80, respectively, all P<0.05), FEF 25%pred ( r values were -0.47, -0.39, and -0.74, all P<0.05) were negatively correlated.There was no direct correlation between R5-R20 and FEV 1%pred, FEV 3%pred, FEF 75%pred, FEF 50%pred, FEF 25%pred (all P>0.05). Conclusions:IOS delivers certain clinical value in evaluating the small airway dysfunction in patients with COPD.Fres is strongly correlated with X5 and conventional lung function indexes, and it is eligible to serve as common means for clinical evaluation.