Pneumonia is a common inflammatory lung disease. Circular RNA (circRNA) vacuolar ATPase assembly factor (circ_VMA21) has been reported to mitigate the inflammatory injury in WI-38 cells. This study was to investigate the functional mechanism of circ_VMA21. Cell model was established by lipopolysaccharide (LPS) treatment in WI-38 cells. Cell cycle and apoptosis were analyzed by flow cytometry. Cell proliferation was assessed by colony formation and MTT assays. The expression quantification of circ_VMA21, microRNA-409-3p (miR-409-3p) and Kruppel-like transcription factor 4 (KLF4) was performed by qRT-PCR method. The expression of relative protein was detected by Western blot. Inflammatory response was evaluated by ELISA. The target binding was validated by dual-luciferase reporter assay. Cellular analysis indicated that LPS repressed cell cycle and proliferation but induced apoptosis. circ_VMA21 was downregulated in pneumonia samples and LPS-treated WI-38 cells. Functionally, circ_VMA21 assuaged the LPS-induced apoptotic and inflammatory damages. In addition, circ_VMA21 directly targeted miR-409-3p and its function was dependent on the sponge effect on miR-409-3p. Also, KLF4 was a target of miR-409-3p and miR-409-3p inhibitor attenuated LPS-induced cell injury by upregulating KLF4. Moreover, KLF4 was upregulated by circ_VMA21/miR-409-3p axis. These findings suggested that circ_VMA21 relieved the LPS-induced apoptotic and inflammatory injury by the regulation of miR-409-3p/KLF4 axis.