The inner myometrium, also called the junctional zone (JZ), is believed to play a major role in the development of adenomyosis. Recently, we found that the lethal-7a (Let-7a) microRNA (miRNA) was clearly downregulated in the miRNA expression profiles of JZ smooth muscle cells (JZSMCs) of patients with adenomyosis. Lin28, including Lin28A and Lin28B, is responsible for the post-transcriptional downregulation of the Let-7 miRNA family. However, the expression pattern of Lin28 and the function of the Lin28/Let-7 axis in adenomyosis have not yet been identified. In this study, we aim to explore the potential roles of the Lin28/Let-7 axis in the development of adenomyosis. Immunohistochemistry, western blot, and reverse transcription polymerase chain reaction (RT-qPCR) were used to evaluate the Lin28 expression, respectively. The correlation between Let-7a, Lin28A, and Lin28B expression was further examined using Pearson's correlation analysis. RNA interference was used to inhibit Lin28B gene, and then Cell Counting Kit (CCK-8) assay was performed to detect the cell proliferation capacity. The results revealed that the expression levels of Lin28B were upregulated in the JZ of adenomyosis whatever about proteins or mRNA (P < 0.0001); furthermore, its mRNA expression level was negatively correlated with Let-7a (r = - 0.749, P < 0.0001). After inhibiting Lin28B gene, the proliferation capacity of JZSMCs in adenomyosis group decreased after 48 h (P < 0.05). These results indicated that Lin28B may be involved in the pathogenesis of adenomyosis by promoting the proliferation capacity of JZSMCs via regulating Let-7a.