Objective: To investigate the effects of umbilical cord-derived mesenchymal stem cells (UC-MSCs) on apoptosis and proliferation of human lung adenocarcinoma A549 cells via PI3K/AKT signaling pathway, and to explore the mechanism. Methods: UCMSCs were isolated from human umbilical cord tissues by enzyme digestion method and cultured in vitro. The immunophenotypes of the obtained MSCs were identified by flow cytometry. The culture supernatant of UC-MSCs was collected to establish an indirect in vitro co-culture system of UC-MSCs conditioned medium and lung adenocarcinomaA549 cell line. Proliferation ofA549 cells was detected by CCK-8 assay; apoptosis ofA549 cells was determined byAnnexin V/PI double staining, and cell cycle distribution of tumor cells was determined by PI staining. The transcription levels of apoptosis and proliferation associated downstream genes in the PI3K/AKT pathway, such as CyclinD1, BAX and Bcl-2, were detected by quantitative polymerase chain reaction (qPCR). Moreover, Wb was utilized to detect the expression levels of PI3K/AKT pathway-related proteins. Results: The culture flask was filled with fibroblast-like cells, arranged in parallel, with spiral growth after three weeks of isolation and culture of human umbilical cord tissues. The flow cytometry results revealed that the MSC markers CD73, CD90 and CD105, but not CD45 and HLA-DR, were expressed on obtained cells.After indirect in vitro co-culture of UC-MSCs conditioned medium and lung adenocarcinoma A549 cells, the proliferation rate of A549 cells was significantly decreased; the apoptosis rate was significantly increased, and the cell cycle was obviously arrested at the G1 phase as compared with the control group (all P<0.01). The transcription levels of PI3K/AKT signaling pathway-related factors, CyclinD1 and Bcl-2 were down-regulated, and the transcription level of BAX was up-regulated (all P<0.01). The total AKT was not changed, but p-AKT protein expression was decreased in a dose-dependent manner inA549 cells cultured in UC-MSCs conditioned medium (P<0.01). Conclusion: UC-MSCs can affect the proliferation and the apoptosis of A549 cells, and arrest cells in G1 phase. The main mechanism is that UC-MSCs can inhibit the PI3K/AKT signaling pathway in A549 cells, providing an experimental basis for exploring the safety and effectiveness of clinical application of UC-MSCs.