Sour and smelly baby’s mouth may be caused by not cleaning the mouth in time, it may also be caused by indigestion, and it may also be caused by diseases such as phenylketonuria. 1. Physiological factors: if the baby did not rinse or clean the mouth in time after eating, the bacteria multiply and decompose the remaining food debris, there can be the phenomenon of the baby’s mouth acidic odor, which is a normal phenomenon, just need to help the baby rinse to alleviate the mouth. 2. indigestion: if the baby has indigestion, the food eaten for a long time to stay in the stomach decomposition, fermentation, can not empty into the intestinal tract digestion, absorption, in this case, the gas produced in the stomach can flow back up, leading to the baby’s mouth sour symptoms. 3. phenylketonuria: phenylketonuria is an autosomal recessive amino acid metabolic disease, babies with this disease can also appear in the mouth of the phenomenon of sour odor, in addition, the disease of the baby’s sweat, urine will appear in the smell of rat urine, the baby may also appear on the development of intellectual abnormalities. Excluding physiological factors, if the baby’s sour mouth symptoms continue to be unrelieved, you should consult a doctor to identify the cause and ask the doctor to deal with it.