Placenta serves as a temporary fetal organ, which mediates maternal-fetal crosstalk and intrauterine fetal growth. Placental defensive barrier is a fundamental physiological function, which balances maternal immune tolerance to the fetus and resistance to pathogens. This review summarizes the latest research progress on the mechanisms of placental barrier formation from the view of placental development. Recent discoveries have shed light on the cellular and molecular properties of placental defensive mechanisms in syncytiotrophoblast, including autophagy, exosome mediated anti-pathogenic pathways, cell-cell junctions and cytoskeleton networks. We also present an overview of placental barrier dysfunction and its implications in intrauterine TORCH infections.