Breast milk (human milk) is natural food to feed human newborns. It is not only the source of energy and nutrients for infant, but also a kind of comprehensive extension protection provided by the mother for babies. As a result of natural evolution, breast milk has diverse and complex components, including nutrients that support infant metabolism, and bioactive substances that help infant development. The content, composition of subcomponents, and chemical and biological status of these components in breast milk can perfectly match the infant's characteristics of anatomy, physiology, biochemistry, and meet the requirements of energy and nutrition, immune function, microecological development, and other needs, which is a biological necessary system for survival and healthy growth of infants. The composition of breast milk is also a foundation of the health superiority of breastfeeding. Due to the natural compatibility between the breast milk composition and the needs of the infant, breastfeeding shows a protective effect on the healthy growth and development of the infant in early life and reduces the risk of chronic diseases in adulthood. Breastfeeding can ensure to improve a healthier physical growth of the breast-fed infants, promote brain growth and cognitive development, balance the development of immune system, improve resistance, reduce the risk of infectious diseases, and provide a significant protective effect on allergic diseases. In terms of long-term health benefits, breastfeeding reduces the risk of obesity in childhood and adulthood, and significantly reduces the risk of metabolic syndrome (hypertension, dyslipidemia and insulin resistance) in adulthood. Further research on the composition of breast milk is not only helpful to understand the nutritional needs of infants, but also is an indispensable scientific support for the development of infant formula food.