Abstract: 　Objective　To investigate the clinical features of Ureaplasma urealyticum (UU) infection in preterm infants under 34 weeks’ gestation. Methods　A total of 232 preterm infants under 34 weeks’ gestation were enrolled. The UU in the lower respiratory tract secretions from infants within 24 hours of admission were detected by real-time fluorescence quantitative polymerase chain reaction. According to the results, infants were divided into positive and negative groups. The differences of the clinical features were compared between two groups. Results　Ninety-seven infants were UU positive in 232 preterm infants. The gestational age and birth weight of the positive group were lower than those of negative group (all P<0.01). The difference in sex, maternal gestational hypertension, prenatal use of hormones and perinatal asphyxia between two groups were not significant (P>0.05). The rates of histologic chorioamnionitis, premature rupture of membranes and vaginal delivery in the positive group were higher than those in the negative group (P<0.01). At 24 and 72 hours after birth, the number of leukocytes and neutrophils in the positive group were significantly higher than those in the negative group (P<0.05). One week after birth, the lung injury was more serious in the positive group than that in the negative group (P<0.05). The rates of re-ventilation during ventilator weaning and needs of ventilator after 10 days were higher in the positive group than those in the negative group. At 28 days after birth, the rate of the bronchial dysplasia was higher in the positive group than that in the negative group (P< 0.05) and the rate of acute respiratory distress syndrome was lower in the positive group than that in the negative group (P<0.05). The rates of retinopathy, periventricular-intraventricular hemorrhage, periventricular leukomalacia, neonatal necrotizing enterocolitis and extrauterine growth retardation were not different between the two groups (all P<0.01). Conclusions　The presence of UU in lower respiratory tract secretions in preterm infants under 34 weeks’ gestation may be associated with premature birth and low birth weight, and may cause chronic lung injury.