Permanent teeth must be extracted as soon as they emerge to avoid malocclusion. When the permanent teeth erupt, the roots of the milk teeth will be resorbed. The root of the milk tooth can fall off by itself after resorption. Under normal circumstances, the milk tooth falls off first and the permanent tooth erupts after the milk tooth is in its original position. If the permanent teeth deviate from the normal eruption path during eruption, or if there is a fistula in the apical root of the milk teeth, the permanent teeth erupt in the direction of the fistula, and the milk teeth do not fall out, the retained milk teeth need to be removed as soon as possible, and the permanent teeth can be returned to the normal position to a large extent. After the eruption of permanent teeth, milk teeth must be extracted as soon as possible, most of the permanent teeth can return to the normal position, and more hard things during the replacement period will help to inherit the eruption of permanent teeth.