Twin-Block orthodontic appliance is a functional orthodontic appliance invented by Clark in 1973, through the functional advancement of the lower jaw, the jaw force is transmitted to the oblique surface of the jaw pad covering the posterior teeth, which changes the direction of the oblique surface bearing the jaw force in the natural tooth series, generating the jaw force that is conducive to normal jaw facial growth, stimulating the growth of the condyle and promoting the reconstruction of the TMJ joint fossa through the advancement of the lower jaw, which is a reasonable orthodontic method in accordance with the physiological characteristics of the growing It is a reasonable orthodontic method that meets the physiological characteristics of patients during the growth period. When Twin-Block keeps the jaw in the anterior extension position, the condyle is displaced to the anterior inferior, and the articular disc ligament connecting the posterior part of the condyle to the articular fossa is stretched and affects the surrounding tissues. Functional orthodontic appliances are mainly used in children during the period of vigorous growth and mixed dentition, which is a better treatment period, and can also be used in the early permanent dentition, because functional orthodontic appliances affect bone growth and dentition development by changing neuromuscular activity. Therefore, when facial growth is nearing completion, the efficacy of functional orthodontic appliances is limited. Functional orthodontic appliances can be used for malocclusion caused by abnormal orofacial muscle function to promote normal functional activity and provide a favorable environment for maxillofacial bone and dentition. It is mainly used for the treatment of Angle Class II mandibular recession, functional anticuspid, crossbite, deep overlap and An’s Class III malocclusion, etc. It can also be used to maintain the effect of treatment, but is not applicable to cases of crowding, misaligned teeth and tooth extraction.