Objective To study the effect of intestinal microecology on postnatal weight gain of very preterm infants in neonatal intensive care unit (NICU). Methods Very preterm infants who met the inclusion criteria were enrolled. The subjects were divided into the extrauterine growth retardation (EUGR) group(defined as a body weight less than the 10th percentile of the corresponding gestational age or a weight loss between birth and a given time of > 2SD were considered EUGR) and normal growth group, and the growth was evaluated at 2 and 4 weeks after birth. Meanwhile, the stool samples were taken to perform16S ribosomal RNA (rRNA) high -throughput 16S rRNA sequencing of the intestinal microflora was performed on stool samples. Results A total of 22 infants were included. There was no significant difference in the alpha diversity indexes indices between the two groups at 2 weeks or 4 weeks after birth. The beta diversity analysis showed that the two groups had similar principal components of the intestinal microflora were similar between the two groups. Linear discriminant analysis (LDA) effect size (LEfSe) showed that 2 weeks after birth, the bacteria with an absolute LDA score (log10) higher than 4 included Streptococcaceae, Streptococcus, Bacteroidetes, Bacteroidales and Stenotrophomonas in the EUGR group and Enterococcaceae and Enterococcus in the control group. At the 4th week after birth, the bacteria with an absolute LDA score (log10) higher than 3 in the EUGR group includedwere Clostriaceae, Eubacteriaceae and Eubacterium. TheBy comparing the composition of the microbial community composition comparison showed, significant differences were found in the principal components of Enterococcus and Streptococcus on the family and genus levels at 2 weeks after birth. No Bifidobacterium was found in either group at 4 weeks after birth. Conclusion Intestinal microecology is different between infants with EUGR and those with normal growth. The diversity and richness of the intestinal microflora in preterm infants at the NICU are significantly insufficient and change dynamically with time, and the establishment of intestinal homeostasis is obviously delayed.