Breast milk diarrhea is due to congenital lactase deficiency or insufficient activity or enteritis leading to secondary lactase deficiency, resulting in incomplete digestion and decomposition of lactose in breast milk, thus causing diarrhea, which is manifested by frothy and dilute watery stools, sometimes containing milk flaps, occasionally green, with streaks of clear mucus, and a special sour odor. Generally, the number of stools is high, ranging from 3 to 20 times a day. A few babies experience vomiting, and in severe cases, dehydration and acidosis can occur. The diagnosis of breast milk diarrhea can be confirmed by stool reducing sugar and pH measurement. If the baby is in good spirits, eats enough milk and has normal weight gain, this condition does not require special treatment. However, if breastfeeding diarrhea has a high number of stools and affects the normal increase in length and weight of the baby or if the growth is slow, this condition needs to be treated with dietary modification. The treatment is to start with lactose-free formula, including milk-based lactose-free formula and soy-based formula, and then gradually increase the number of breastfeeding sessions and switch to a mixture of breast milk and lactose-free formula according to the child’s tolerance after the diarrhea stops. Mothers who breastfeed in their daily life should also pay attention to their diet. If the mother often eats foods with a lot of oil, high fat and stimulants, resulting in the milk containing more ingredients than the baby needs, it may cause the baby to develop diarrhea. In addition, the mother should avoid eating foods that can cause allergies in the baby. For example, if the baby is allergic to milk, eggs, seafood, wheat, etc., the mother may also experience increased diarrhea or even blood in the stool if the baby eats these foods. However, if the baby has sudden onset of diarrhea within a short period of time, with a significant increase in the number of stools or mucus-purulent stools, along with a significant decrease in appetite, frequent vomiting, decreased urine output, frequent crying, fever, poor weight gain, and eczema, this is to be considered as other causes of diarrhea and requires prompt medical treatment.