Parents are often puzzled and annoyed by the fact that their children’s replacement permanent teeth are not as neat and beautiful as their milk teeth. Malocclusion is mainly caused by two major factors: heredity and environment, which act on bones, muscles and teeth and make them change. 1, genetic factors refers to the sperm cells and egg cells at the time of conception has been passed on to the child by the role of genes. Malocclusion is a high proportion of the causes of malocclusion, and the role of genetic factors is generally gradually apparent in the replacement of milk teeth. Common genetic factors cause malocclusion are facial asymmetry, sparse teeth, crowded teeth, abnormal number of teeth, morphology, eruption time, maxillary protrusion, mandibular protrusion, mandibular retrusion and deep cover. 2.Environmental factors include both congenital and acquired factors. Firstly, congenital factors refer to the development of fetal bone calcification, teeth and organs in the mother’s body due to the mother’s malnutrition, illness, trauma, radium and high-dose radiation, which affects the development of fetal bone calcification, teeth and organs. It often manifests itself in the following ways: anterior reverse, open, multiple teeth, congenitally missing teeth, oversized or undersized teeth, conical teeth, fused teeth, and tooth surface defects. Secondly, the acquired factors refers to the fetus after birth, due to certain acute and chronic diseases, endocrine function abnormalities (thyroid, pituitary gland dysfunction), malnutrition, a variety of oral bad habits, early loss of milk teeth, milk teeth stagnation, early loss of permanent teeth, early eruption of permanent teeth, permanent teeth eruption order disorders, the milk cusp of the teeth wear insufficient malformations can be caused. The formation factors of malocclusion are intricate, related to each other, difficult to distinguish between primary and secondary, and interact with each other in their roles.