Objective: To study the timing of presentation and perinatal high-risk factors for necrotizing enterocolitis (NEC) in preterm infants with a gestational age of <33 weeks. Methods: A case-control study was conducted in 49 preterm infants with NEC (gestational age <33 weeks) who were admitted to the Neonatal Intensive Care Unit of Beijing Bayi Children's Hospital between October 1, 2010 and December 30, 2012, as well as preterm infants without NEC during the same period. The timing of presentation of NEC was retrospectively analyzed, and the perinatal high-risk factors for NEC were determined by multivariate logistic regression analysis. Results: The median age of onset was 17.5 days (range: 3-106 d) in preterm infants with NEC. Sex, being small for gestational age, delivery mode and antenatal corticosteroid therapy were not associated with the development of NEC; low gestational age, low birth weight and neonatal asphyxia increased the risk of NEC, and low gestational age was identified as an independent high-risk factor for the development of NEC. Conclusions: Low gestational age is an important risk factor for the development NEC in preterm infants under 33 weeks' gestation, and the median age of onset is 17.5 days.