Objective To investigate the dynamic development of gut microbiota in preterm infants during their first year after birth and to compare the microbiota profiles between preterm infants of different gestational ages. Methods In this prospective study, 81 preterm infants (including 37 early preterm and 44 moderate-to-late preterm infants) and 26 full-term infants were enrolled from Shenzhen Bao'an Women and Children's Hospital between January 2021 and September 2022. Fecal samples were collected at 1, 6, and 12 months of age. 16S rRNA sequencing was conducted, followed by comprehensive bioinformatics analysis. Results The alpha diversity indices of the gut microbiota in preterm infants increased significantly from 1 to 12 months of age (P<0.001). With increasing age, the relative abundances of Clostridium, Streptococcus, Klebsiella, and Staphylococcus decreased, whereas those of Bifidobacterium, Lactobacillus, and Bacteroides increased. At 1 month of age, significant disparities were observed in alpha diversity, beta diversity, and taxonomic abundances among different gestational age groups. Specifically, Clostridium and Aeromonas were enriched in early preterm infants. By 12 months of age, no significant differences in these microbial metrics were detected across the gestational age groups (P>0.05). Conclusions The gut microbiota of preterm infants becomes progressively more diverse during the first year of life. Preterm infants with lower gestational age harbor a higher relative abundance of potentially pathogenic genera in their early gut microbiota. However, the influence of gestational age on microbial composition diminishes over time.