Breast milk is the best food source for infants. It contains rich proteins, lipids, carbohydrates and other nutrients as well as complex immunoglobulin (Ig) and hormones and other biologically active substances, which not only meet the growth and development needs of infants, but also promote development and improvement of their immune and nervous systems etc. of infants. The composition of breast milk is different due to influence of dietary habits and health status, gestational age at delivery and other factors of mothers of infants. The article intends to elaborate on the latest research progresses of composition of breast milk and its related influencing factors and benefits of breast feeding for preterm infants.