Breast milk is the best natural food for babies, and sufficient breast milk can meet the needs of growth and development of babies up to 6 months old. However, some babies have diarrhea when they drink breast milk, and they have diarrhea while eating. Mothers worry that their children are suffering from enteritis, and they have frequent stool tests and even take various drugs, but their babies still have diarrhea, in fact, they are lactose intolerant. Lactose is one of the sugars and is one of the main nutrients for the growth and development of babies. Lactose is contained in milk and lactase exists on the surface of the mucous membrane of the small intestine, which can break down lactose into glucose and galactose for our body to absorb and use. If lactase secretion decreases, it cannot completely digest and decompose lactose in milk, so that part of lactose is fermented by colonic flora into lactic acid, carbon dioxide and other gases, lactic acid stimulates the intestinal wall, increases intestinal osmotic pressure, increases intestinal peristalsis, and the baby will have diarrhea, flatulence, colic crying, etc. If the baby does not have a lot of stools and the weight gain is normal, the mother adjusts the diet and takes oral intestinal microecological preparations. If it does not relieve and affects the baby’s weight gain, lactase can be supplemented before breastfeeding, or special formula without lactose can be replaced. Congenital lactase deficiency is rare, most of them are secondary to reduced lactase secretion, and as the baby’s gastrointestinal digestive function improves, he or she can continue to drink breast milk or other milk types. Therefore, when a newborn baby has breast milk diarrhea and parents are not sure of the cause, it is recommended to seek medical attention and feed under the guidance of a medical professional, and not to stop breast milk or take medication without authorization.