Intraoperative transesophageal echocardiography is used to more accurately assess a patient's cardiac condition and is important for the guidance of cardiac surgery. This case reports a pediatric cardiac surgery performed under TEE guidance in a patient with multiple complications. A 5-year-old child presented with diarrhea as the first symptom. At the age of 2 years, she was admitted to the hospital with ptosis of the right upper eyelid, morning lightness and evening heaviness, hand tremor, and the presence of a murmur in the mid to late stages of cardiac systole on auscultation. She was diagnosed with mitral valve prolapse and myasthenia gravis, which resolved with treatment. Subsequently, she was diagnosed with hyperthyroidism due to obvious protrusion of the eyeballs, and was admitted to the hospital for heart valve surgery after her thyroid function was controlled. Intraoperative TEE suggested mitral valve prolapse combined with embolization of the P2 region. In the complex case of this child, the suture points during mitral valvuloplasty and the scar tissue caused by the mitral valvuloplasty annulus may limit the growth of the annulus as well as the leaflet motion. Therefore, it was recommended that the surgeon perform mitral valve extension to maximize the prognosis. TEE herein guided the cardiac surgical decision making to maximize the prognosis of the child.