Objective：To explore the expression of microRNA let - 7 in the serum of stable male COPD patients and its clinical significance. Methods：One hundred and sixty COPD patients in stable period and 100 healthy volunteers were selected to collect clinical data and plasma samples. Serum samples were extracted by centrifugation at 4 ℃. The relative expression levels of let-7a，let- 7c and let -7d in serum of each group were detected by qPCR. Correlation analysis was conducted between let - 7 and lung function indexes. The ROC curve was used to analyze the diagnostic value of let-7 for COPD. Results：qPCR results showed that the serum let- 7a，let-7c and let-7d levels in COPD patients were significantly lower than the control and smoking groups(P ＜ 0.05). The expression of let-7 in GOLD groups had no significant difference(P ＞ 0.05). The expression levels of let-7 in the serum of the smoking group were lower than those of the control group，but the differences were not statistically significant(P ＞ 0.05). Pearson analysis showed that the serum let-7 expression had positively correlation with lung function(FEV1/FVC，MEF75%pred，MEF50%pred，MEF25%pred，MMEF75/25% pred)(P ＜ 0.05)，among which let-7 had the strongest correlation with MMEF75/25%pred；let-7c was positively correlated with FEV1% pred(P ＜ 0.05)，while let-7a and let-7d were not significantly correlated with FEV1%pred(P ＞ 0.05). ROC curve analysis showed that the area under the ROC curve of let-7a，let-7c，let-7d and combined index were 0.808，0.708，0.761，0.828，respectively，the diagnostic efficiency of combined indexes was better，and its sensitivity and specificity were 83.51% and 70.51%，respectively. Conclusion：The decrease of the serum let -7 expression level was related to the incidence of COPD. Let -7 has diagnostic value in patients with mild COPD，which could provide reference for timely clinical diagnosis and standardized treatment.