Background: Bronchopulmonary dysplasia (BPD) is one of the most common adverse consequence of premature delivery and the most common chronic lung disease in infants. BPD is associated with long-term lung diseases and neurodevelopmental disorders that can persist into the adulthood. The adverse consequences caused by severe BPD are more serious. However, there were few studies on the risk factors for severe BPD. Methods: This is a retrospective study of preterm infants born less than 32-week gestational age (GA) and diagnosed with BPD. Results: A total of 250 preterm infants with a diagnosis of BPD and GA <32 weeks were included (137 boys [54.8%] and 113 girls [45.2%]). The birth weight ranged from 700g to 2010g and the mean birth weight was 1318.52g (255.45 g). The GA ranged from 25 weeks to 31 weeks and 6days (mean, 30weeks). The number of cases of mild, moderate and severe BPD were 39 (15.6%), 185 (74.0%) and 26 (10.4%), respectively. There were significant differences in the rate of small for gestational age (SGA), intrauterine asphyxia, pulmonary hemorrhage, neonatal respiratory distress syndrome (NRDS), circulatory failure, pulmonary hypertension, patent ductus arteriosus (PDA), pulmonary surfactant (PS), aminophylline, caffeine, glucocorticoids, tracheal intubation, diuretics, and parenteral nutrition length among the three groups (P < 0.05). The time of parenteral nutrition (aOR = 3.343, 95%CI: 2.198 similar to 5.085) and PDA (aOR =9.441, 95%CI: 1.1867 similar to 5.128) were independent risk factors for severe BPD compared with mild BPD. PDA (aOR = 5.202, 95%CI: 1.803 similar to 15.010) and aminophylline (aOR = 6.179, 95%CI: 2.200 similar to 17.353) were independent risk factors for severe BPD, while caffeine (aOR = 0.260, 95%CI: 0.092 similar to 0.736) was the protective factor for severe BPD compared with moderate BPD. The time of parenteral nutrition (aOR = 2.972, 95%CI: 1.989 similar to 4.440) and caffeine (aOR =4.525, 95%CI: 1.042 similar to 19.649) were independent risk factors for moderate BPD compared with mild BPD. Caffeine (aOR = 3.850, 95%CI: 1.358 similar to 10.916) was the independent risk factor for moderate BPD, while PDA (aOR = 0.192, 95%CI: 0.067 similar to 0.555) and aminophylline (aOR = 0.162, 95%CI: 0.058 similar to 0.455) were protective factors for moderate BPD compared with severe BPD. The time of parenteral nutrition (aOR = 0.337, 95%CI: 0.225 similar to 0.503) and caffeine (aOR = 0.221, 95%CI: 0.051 similar to 0.960) were protective factors for mild BPD compared with moderate BPD. The time of parenteral nutrition (aOR = 0.299, 95%CI: 0.197 similar to 0.455) and PDA (aOR = 0.106, 95%CI: 0.013 similar to 0.843) were protective factors for mild BPD compared with severe BPD. Conclusion: The time of parenteral nutrition is the risk factor of moderate and severe BPD. PDA and aminophylline are risk factors for severe BPD. The role of caffeine in the severity of BPD is uncertain, and SGA is not related to the severity of BPD. Severe or moderate BPD can be avoided by shortening duration of parenteral nutrition, early treatment of PDA, reducing use of aminophylline and rational use of caffeine.