There may be the following reasons for a 4-month-old baby’s ear pus: First, the child’s mother may have the wrong posture when breastfeeding the child, because the child’s eustachian tube is short, flat and straight, so if the breastfeeding posture is not correct, it will easily lead to milk entering the middle ear cavity through the eustachian tube in the nasopharynx, thus inducing otitis media and thus pus in the ear canal; Second, the child may have a cold or some other upper respiratory tract infection. These viruses and bacteria enter the middle ear cavity through the eustachian tube, thus inducing purulent otitis media; third, some parents of children may enter the outer ear canal when bathing their children, resulting in some infection inside the outer ear canal, thus inducing otitis media. In conclusion, if a four-month-old baby has pus in the ear, parents are advised to take the child to an ENT clinic as soon as possible to check the condition of the eardrum. Clinically, if otitis media is diagnosed, the ear canal should be cleaned with hydrogen peroxide drops and anti-inflammatory treatment should be given.