Objective To investigate the clinical characteristics and risk factors for early-onset necrotizing enterocolitis (NEC) in preterm infants with very/extremely low birth weight (VLBW/ELBW). Methods A retrospective analysis was performed on the medical data of 194 VLBW/ELBW preterm infants with NEC who were admitted to Children's Hospital Affiliated to Zhengzhou University from January 2014 to December 2021. These infants were divided into early-onset group (onset in the first two weeks of life; n=62) and late-onset group (onset two weeks after birth; n=132) based on their onset time. The two groups were compared in terms of perinatal conditions, clinical characteristics, laboratory examination results, and clinical outcomes. Sixty-two non-NEC infants with similar gestational age and birth weight who were hospitalized at the same period as these NEC preterm infants were selected as the control group. The risk factors for the development of early-onset NEC were identified using multivariate logistic regression analysis. Results Compared with the late-onset group, the early-onset group had significantly higher proportions of infants with 1-minute Apgar score ≤3, stage III NEC, surgical intervention, grade ≥3 intraventricular hemorrhage, apnea, and fever or hypothermia (P<0.05). The multivariate logistic regression analysis showed that feeding intolerance, blood culture-positive early-onset sepsis, severe anemia, and hemodynamically significant patent ductus arteriosus were independent risk factors for the development of early-onset NEC in VLBW/ELBW preterm infants (P<0.05). Conclusions VLBW/ELBW preterm infants with early-onset NEC have more severe conditions compared with those with late-onset NEC. Neonates with feeding intolerance, blood culture-positive early-onset sepsis, severe anemia, or hemodynamically significant patent ductus arteriosus have a higher risk of early-onset NEC.