The oral midline is usually defined as the line in the sagittal plane of the oral cavity based on the line connecting the upper lip bead, the upper and middle incisors, and the lower and middle incisors, which is the oral midline. In oral anatomy, the use of dental anatomy, the human center, brow, chin and the entire facial soft tissues to determine the midline of the face, the midline as a sagittal plane, usually the upper lip bead, upper middle incisor crevice, lower middle incisor crevice line for the oral midline. Teeth erupt symmetrically in the midline, and the number is consistent. When the teeth in the mouth are crowded, the upper and lower incisors cannot be aligned, and then the facial midline is used as a reference to determine the oral midline, and the facial midline and oral midline coincide in the sagittal plane of the human body. When teeth are missing and chipped in the mouth, the midline needs to be used as a standard to determine the tooth position. During the orthodontic treatment of crowding, the midline is also used as a criterion to determine the direction and position of the orthodontic target.