Bronchopulmonary dysplasia (BPD) is a chronic lung disease, which is a major complication of very low and ultra-low preterm infants. Moderate and severe BPD survivors are prone to adverse outcomes such as impaired lung function, childhood exercise intolerance, and neurodevelopmental retardation in the long term, which seriously affects their quality of life and brings a heavy burden to society and families. However, the pathogenesis of BPD is complex, including pulmonary vascular dysplasia, lung inflammation, and impaired alveolar development. There is currently no specific clinical drug to cure BPD. Mesenchymal stem cells (MSCs) are a kind of multipotent stem cells that exist in almost all organs and tissues of individuals. MSCs have the properties including self-renewal, multi-directional differentiation, and immunosuppressive and anti-inflammatory abilities. Preclinical studies have shown that MSCs can alleviate BPD by improving alveolar and pulmonary vascular development, and reducing pulmonary fibrosis. Several phase I clinical studies have demonstrated that intratracheal transplantation of human umbilical cord blood-derived mesenchymal stem cells for children with BPD is safe and feasible. This study aims to further evaluate the safety and efficacy of umbilical cord-derived mesenchymal stem cell transplantation in the treatment of severe BPD in premature infants, in the hope of increasing the survival rate and improving the prognosis of severe BPD.