Healthy newborns develop metabolic acidosis 2 to 3 weeks after birth, which is called late metabolic acidosis of the newborn. Its onset is related to the quality and quantity of protein in the diet, as well as imperfect renal function and excessive renal acid load. Typical late metabolic acidosis can occur in premature infants without other problems in the second to third week after birth. It is characterized by poor weight gain related to metabolic acidosis (BE<-8mmol/L) (daily weight gain <10g). These children may be asymptomatic or only show excitement. Their BE value is usually between -10 and -16mmol/L. It may also affect the growth and development of children and their body resistance, and even endanger their lives due to complications of other serious diseases. Late metabolic acidosis is one of the most common metabolic problems in newborns, especially premature infants. It is more common in premature infants and low-birth-weight babies. The incidence of late metabolic acidosis in premature infants is significantly higher than that in full-term infants. The incidence is even higher in extremely low birth weight infants with a birth weight of ≤1500g. The younger the gestational age of the premature baby, the lower the birth weight, and the longer the duration of metabolic acidosis.