The extra-oral arch is a device that connects the inside and outside of the mouth and transmits extra-oral forces to the inside of the mouth and consists of an intra-oral and extra-oral arch. The intraoral arch is connected to the intraoral orthodontic appliance through a buccal tube inserted into the ring of the molar, while the extraoral arch is connected to the head cap or cervical band through an elastic rubber band or spring to generate orthodontic forces. The main functions of the extra-oral arch are to strengthen the support (to prevent proximity of the molar), to push the molar towards the distal center, to depress the molar, and to widen or narrow the arch. Its greatest advantage is that it exerts forces on intraoral teeth with the help of extra-oral cranial support and does not exert forces on non-acting teeth. The extra-oral arch is most commonly used in patients with maxillary protrusion deformity, which is manifested by lip tilting of the upper front teeth, open lips and exposed teeth, affecting the aesthetics. Such patients often have a strong desire to improve the anterior protrusion of the upper front teeth. Usually we use extractive orthodontic treatment to retract the protruding upper front teeth as much as possible. In contrast, the general non-extraction orthodontic treatment uses the back molars as support to retract the upper front teeth inward, and under the reaction force, the front teeth retract inward while the back molars also move forward. In order to keep the molar from moving forward and to retract the upper anterior teeth as much as possible, extra-oral support is often used clinically, i.e., an extra-oral arch is used to prevent the molar from moving closer.