High-frequency ultrasound was used to examine the cause of neck muscle swelling and hardening by analyzing the morphology, internal echo structure and blood supply of the affected sternocleidomastoid muscle and comparing it with the healthy sternocleidomastoid muscle. High-frequency ultrasound can clearly show the structure of the sternocleidomastoid muscle, and the affected side of the sternocleidomastoid muscle is mostly pike-shaped enlargement, and the internal echoes are mostly inhomogeneous and slightly hypoechoic, with muscle texture and some blood flow signals, which are obviously different from the healthy side. Color Doppler ultrasound diagnostic equipment is used, and children generally do not need special preparation before the examination, and the examination is most effective after falling asleep. The examination is performed in the supine position, and the crying child can be held diagonally by the mother, with the neck slightly extended backward and rotated to the healthy side to fully expose the neck. Multiple consecutive sections along the course of the sternocleidomastoid muscle are scanned to observe the morphology of the sternocleidomastoid muscle on the affected side, its internal echogenicity, and its relationship with the surrounding tissues. The thickness of the sternocleidomastoid muscle is measured bilaterally, and then color Doppler is used to observe the blood flow in and around the mass, and spectral Doppler is used for measurement and analysis. This condition is relatively common in children, with an incidence of 0.008% to 0.04%, and is rarely seen at birth. A mass appears in the lower part of the sternocleidomastoid muscle at 1 week, is firm and fixed, and rotates the head to the contralateral side at 2 to 3 weeks, with seemingly restricted neck movement. The etiology of the disease is still not completely clear, the earliest theory is that the venous return in the muscle is obstructed, and some people think that a birth injury causes muscle rupture and produces blood clots, but none of them have been proved, and some people think that fibrosis of the scimitar is a congenital abnormality or developmental insufficiency, but it needs to be further proved. If the disease can be diagnosed early and non-surgical treatment is taken, 80% of the prognosis is good. When the sternocleidomastoid muscle has contracture and local deformity, the prognosis is poorer even though surgical treatment is taken. Therefore, early diagnosis and early treatment of cmt is very important. High-frequency ultrasound can clearly show the morphology of the sternocleidomastoid muscle mass, internal echoes and its internal blood supply, which is important for the correct diagnosis of cmt, especially for patients with congenital strabismus who do not have a mass to look for, ultrasound is more meaningful. Clinically, congenital muscular strabismus should be distinguished from ophthalmic strabismus, bony strabismus caused by cervical vertebrae deformity, strabismus caused by acute lymphadenitis in pediatric patients, spasmodic strabismus, psychogenic strabismus, etc. In the past, it was mainly based on clinician’s experience, experience, and experience. In the past, the diagnosis or differential diagnosis was mainly based on the experience of clinicians, cervical spine x-ray films, and the patient’s medical history. Although x-ray film can identify bony cervical strabismus, cervical scoliosis caused by the sternocleidomastoid muscle will affect the diagnosis of x-ray film; acute lymphadenitis in children can rapidly develop cervical strabismus and present with a mass in the neck, but this mass is often with pressure and pain and is not located in the sternocleidomastoid muscle, and high-frequency ultrasonography can detect multiple swollen lymph nodes; spasticity cervical strain and psychogenic strabismus should be diagnosed with a medical history, but the family’s account is subjective and not always easy to be understood by the patient. Spasmodic cervix and mental cervix need to be diagnosed in the context of the patient’s medical history, and the patient’s family is prone to subjective factors in the history, so it is difficult to avoid misdiagnosis if the clinician relies only on his or her experience in making the diagnosis or differential diagnosis. High-frequency ultrasound can easily differentiate cmt from non-myalgic cervical strabismus based on the characteristic images of myalgic strabismus. High-frequency ultrasound clearly demonstrates the extent of the sternocleidomastoid lesion, its internal echogenicity, and its internal blood supply. Ultrasound is valuable in differentiating the type of fibrous degeneration, and it can be used to inform the choice of clinical treatment. If the internal echoes of the sternocleidomastoid muscle are found to be significantly enhanced, i.e., fibrosis is present, surgery should be performed as soon as possible to avoid repeated treatments and delayed treatment time.