Objective: To investigate the phenomenon of small airway epithelial-mesenchymal transition (EMT) in patients with normal pulmonary function and chronic obstructive pulmonary disease (COPD) who received surgical treatment for lung tumors. Methods: In this study, 52 patients undergoing surgical treatment for lung tumors admitted to the General Hospital of Ningxia Medical University were included from January 2018 to January 2019. According to the preoperative pulmonary function and smoking history, patients were divided into group A (non-smoking with normal pulmonary function group, 15 patients), group B (smoking with normal pulmonary function group, 21 patients), group C (COPD stable phase group, 16 patients). HE staining was performed to observe the pathological changes in small airway and lung tissue. Immunohistochemistry was used to detect the localization and expression of E-cadherin, α-smooth muscle actin (α-SMA) and Vimentin in small airway. Western blot was used to detect the levels of E-cadherin, α-SMA and Vimentin. Results: (1) There were no significant differences in age, gender, and the ratio of disease composition among the three groups (P>0.05), but forced expiratory volume in one second (FEV(1)) as percentage of predicted value (FEV(1)%pred) and FEV(1)/forced vital capacity (FEV(1)/FVC) in group C were lower than those in groups A and B (all P<0.01), while there was no significant difference between group A and group B (P>0.05); (2) the bronchial wall thickness in group B and group C were higher than that of group A [(32.4±2.4) and (54.6±4.9) vs (22.4±2.2) μm], and group C was significantly higher than group B (P=0.003); (3) the expression levels of E-cadherin in the epithelial cells of small airway in group B and group C were lower than those in group A (0.021±0.008 and 0.018±0.007 vs 0.062±0.010) (all P<0.05), while the levels of mesenchymal cell markers such as α-SMA and Vimentin in group B and group C were higher than group A, and group C was higher than group B (α-SMA: 0.641±0.113, 0.780±0.133 vs 0.404±0.123; Vimentin: 0.064±0.033, 0.083±0.022 vs 0.030±0.021) (P=0.002 and P=0.003). Conclusion: In patients undergoing surgical treatment of lung tumors, there is EMT in the small airways of patients with COPD, and EMT has occurred in the small airways of smokers with normal pulmonary function.