As enteral nutrition is not feasible soon after birth in most preterm infants, parenteral iron administration is an efficacious method for investigators to select. For most preterm infants, the use of parenteral nutrition(PN) is very common during the first ten days of life, so the investigators hypothesis that iron-fortified PN may have a preventative and treatment effect on preterm infants using PN as a supplementation of oral nutrition; Iron-fortified PN can also improve iron store status of preterm infants. The higher concentration of iron used in this study, the larger preventative or treatment effect on preterm infants anemia; it is safe to add small dose of iron agent to PN.