Objective To investigate differences in gut microbiota between very preterm infants with no/grade I bronchopulmonary dysplasia (BPD) and those with grade II/III BPD. Methods A prospective cohort study was conducted at Women and Children's Hospital, School of Medicine, Xiamen University from January 2023 to June 2024. Preterm infants with gestational age <32 weeks and birth weight <1 500 g were categorized into two groups: no/grade I BPD (n=52) and grade II/III BPD (n=41). Gut microbiota composition was compared using high-throughput sequencing. Results The grade II/III BPD group showed significantly increased relative abundances of Proteobacteria and Klebsiella, and significantly decreased relative abundances of Firmicutes, Bacteroidota, and Staphylococcus. No/grade I BPD infants showed no obvious temporal trend in gut microbiota changes. On postnatal day 21, the relative abundance of Streptococcus was significantly lower in the grade II/III BPD group than in the no/grade I BPD group (P=0.017). At 36 weeks of corrected gestational age, alpha diversity in the grade II/III BPD group was significantly lower than in the no/grade I BPD group (P<0.05). Principal coordinates analysis indicated a significant difference in overall gut microbiota composition between the groups at this time point (P=0.019). Conclusions Very preterm infants with grade Ⅱ/Ⅲ BPD show reduced gut microbial diversity early in life. A decrease in Streptococcus may be associated with grade II/III BPD.