The causes of retained milk teeth are as follows: First, congenital absence of permanent tooth germ, some children are congenitally lack of permanent tooth germ, resulting in the root of milk teeth cannot be absorbed normally, and then the phenomenon of retained milk teeth will occur. If the child’s caries is left unattended, and the infection around the root is secondary, or the trauma of the baby tooth causes embedded injury, it will lead to the necrosis of the permanent tooth embryo and cannot erupt normally. The permanent tooth embryo is misplaced and does not grow under the milk tooth, which leads to the permanent tooth and the milk tooth root can not contact, because the milk tooth root can not be stimulated by the pressure of the permanent tooth, the root does not absorb, so it does not loosen in the dental bed, and the phenomenon of retention occurs. Fourth, children usually eat too fine, chewing stimulation is reduced, resulting in the lack of jaw bone development, which is not conducive to the growth of teeth. Fifth, due to inflammation caused by the root of the milk tooth and alveolar bone adhesion, so that the root of the milk tooth is not completely absorbed or not absorbed at all, the phenomenon of retention of milk teeth.