A sour taste in the mouth of a newborn child may be caused by physiological factors such as improper oral care, or pathological factors such as gastroesophageal reflux. 1. Physiological factors: After the child has finished eating milk, there is no rinsing of the child’s mouth, and the milk remains in the mouth, leading to bacterial growth, and therefore a sour taste in the mouth. 2. Gastroesophageal reflux: Since the cardia and pylorus of the infant’s stomach are almost on the horizontal plane and the cardia sphincter is not well developed, the phenomenon of gastroesophageal reflux is prone to occur, and the milk in the stomach mixes with gastric acid and refluxes into the oral cavity, thus leading to the appearance of a sour taste in the oral cavity. Newborn children with sour taste in the mouth, there may also be other reasons, it is recommended to go to the hospital in a timely manner, improve the examination to clarify the cause of the disease, under the guidance of the doctor to give targeted treatment or treatment.