A four-month-old baby with watery diarrhea needs to be treated with fluid supplementation, using montelukast and bifidobacterium triptans. Diarrhea in a four-month-old baby is related to rotavirus infection and can be diagnosed by means of rotavirus examination. Severe diarrhea can lead to a large loss of water in the body, so attention should be paid to timely replenishment of fluids. For babies with normal food intake, they can take oral rehydration salts to prevent dehydration, and if necessary, they can be improved by intravenous rehydration. Montmorillonite is an anti-diarrheal drug of the adsorption class, which will not be absorbed by the body after entering the digestive system and therefore will not affect the baby’s health. It can also enhance the cohesion of intestinal mucus, adsorb germs and toxins in the intestinal tract and improve the symptoms of diarrhea. Bifidobacterium Triptans is mainly used to supplement intestinal beneficial bacteria, improve the imbalance of intestinal flora, improve the function of the intestinal tract, for diarrhea and constipation have a regulatory effect.