OBJECTIVE To study the incidences of group B streptococcus (GBS) colonization in pregnant women and GBS infection in their preterm infants, and to investigate the risk factors for GBS colonization in preterm infants. METHODS A total of 859 women who delivered before term from January 2017 to January 2018 were enrolled in this prospective cohort study. Bacterial culture was performed for GBS using the swabs collected from the rectum and the lower 1/3 of the vagina of the pregnant women on admission. A total of 515 of the above cases underwent real-time PCR assay for testing of GBS DNA. Bacterial culture was performed for GBS using the oropharyngeal secretion, gastric fluid or blood samples in preterm infants born to the 859 pregnant women. Peripheral blood samples from the pregnant women and umbilical cord blood samples from their preterm infants were collected to determine the level of anti-GBS capsular polysaccharide antibody. The incidence of GBS infection and perinatal risk factors for GBS colonization in the preterm infants were examined. RESULTS The positive rate for GBS in the rectal and vaginal cultures was 14.8% (127/859) among the 859 pregnant women, and the positive rate in the GBS DNA testing was 15.1% (78/515). There were 976 live-birth preterm infants delivered by 859 pregnant women, and 4.4% (43/976) of whom were GBS positive. Four preterm infants had early-onset GBS diseases, including pneumonia in two cases and sepsis in two cases. In 127 preterm infants delivered by 127 GBS-positive pregnant women, the preterm infant group with a gestational age between 34 and 37 weeks had a significantly lower GBS positive rate and a significantly higher level of anti-GBS capsular polysaccharide antibody compared with the preterm infant group with a gestational age of less than 34 weeks (P=0.013 and 0.001 respectively). A multivariate logistic regression analysis revealed that premature rupture of membranes time >18 hours and chorioamnionitis were independent risk factors for GBS colonization in preterm infants (OR=6.556 and 6.160 respectively; P<0.05). CONCLUSIONS GBS positive rate and anti-GBS capsular polysaccharide antibody level in preterm infants are correlated with gestational age. premature rupture of membranes time >18 hours and chorioamnionitis may increase the risk of GBS colonization in preterm infants.