Tooth crowding is a common clinical malocclusion. Mild crowding is often manifested as twisted misalignment of upper and lower incisors or mild heavy misalignment of cuspids, which is usually treated by orthodontic methods of expanding the dental arch, and the crowding is only 2-4 mm. first, the extraoral labial arch can be used to push the molars backwards, and the extraoral labial arch can be used to push the maxillary first permanent molar backwards to obtain a gap to align the mild crowding of the anterior teeth is a common clinical orthodontic method, which is mainly used before the eruption of the second permanent molar. Second, local development gap is commonly used to expand the dental arch to obtain clearance for local development orthodontic techniques of orthodontic treatment methods, mainly for individual teeth crowding misalignment gap shortage, local development of the dental arch gap sources mainly rely on the increase in the arc of the anterior arch; third, available fine wire arch bending a variety of curves to develop the length of the dental arch; fourth, expansion of the orthodontic arch can be divided into split spring slow expansion of the arch also Fourth, orthodontic treatment can be divided into slow expansion with split reeds and fast expansion, and after obtaining a gap, other removable or fixed appliances can be replaced to align the crowded and misaligned teeth. In moderate and severe crowding, the crowding often reaches 4-8 mm or even more than 8 mm. The treatment is done by reducing the number of teeth in conjunction with orthodontic appliances. The selection of extraction orthodontic positions is usually based on the principle that left and right paired diseased teeth are preferred