Severe bronchopulmonary dysplasia (BPD) is a chronic lung disorder that primarily affects premature babies with extremely low birth weight and involves in multiple organ system; no effective pharmacotherapy for this disease exists, and mortality remains high. Based on the evidence from previous preclinical studies and phase I clinical trials, this study aims to test the safety of intravenous application of a single dose of human umbilical cord-derived mesenchymal stem cells (hUC-MSCs) in patients with severe BPD. The Mesenchymal Stem cells for Bronchopulmonary Dysplasia Treatment (MSBDT) trial is a single center, open-label, dose-escalation phase I clinical trial. Severe BPD patients were enrolled in Children Hospital of Chongqing Medical University, Chongqing, China. The first six patients were treated with low-dose hUC-MSCs (1 × 10 cells/kg) and the next seven patients were treated with high-dose hUC-MSCs (5 × 10 cells/kg). This study is registered with ClinicalTrials.gov, number NCT03558334. No prespecified infusion-associated adverse events, immediate complication, respiratory or cardiovascular compromise were observed during infusion and 24 h after infusion. No significant changes in safety laboratory values were observed. One death event occurred in the low-dose group on study day 10, and one death event occurred in the high-dose group on study day 24, while, after review in detail, the two cases are not believed to be infusion-associated events. In conclusion, intravenous application of a single dose of hUC-MSCs was tolerated in thirteen patients with severe BPD.