Narrow dental arch usually refers to the distance between the maxillary first molars <33mm is narrow. There are many causes of dental arch narrowing, mainly caused by bad habits, as follows: 1, breathing, children with rhinitis, sinusitis or adenoid hypertrophy, will often have the habit of mouth breathing, mouth breathing is open, the force of the buccal muscle will increase, the tongue muscle down, loss of balance, resulting in dental arch narrowing; 2, thumb sucking, children sucking thumb, the mouth is open, the force of the buccal muscle increases, the tongue down, resulting in dental arch narrowing; 3, eating food, resulting in increased negative pressure in the mouth, increasing the degree of dental arch narrowing; 3, sucking force, resulting in the arch of teeth. This, together with the sucking force, leads to an increase in negative pressure in the mouth, increasing the degree of narrowing of the dental arch; 3, eating fine food, the jaw bone lacks effective stimulation, leading to narrowing of the dental arch; 4, developmental malformations, such as cleft lip and palate patients, because the cleft lip and palate leads to narrowing of the dental arch; 5, trauma, which also causes narrowing of the dental arch.