Ultrasound features of undifferentiated carcinoma of thyroid mainly include irregular morphology, hypoechoic solid nodule, unclear border and rich blood supply. Undifferentiated thyroid cancer has general features of malignant tumors, such as irregular morphology, unclear border, rich blood supply, etc. It is not different from other types of thyroid cancer. Undifferentiated carcinoma of thyroid also has different ultrasound signs from papillary carcinoma of thyroid and follicular adenocarcinoma. Undifferentiated carcinoma of thyroid is mostly irregular hypoechoic solid nodules with uneven internal echogenicity, but cystic degeneration is rare, and the few cases with cystic component are mostly necrotic hemorrhage within the lesion. Undifferentiated carcinoma is usually without calcification, and the aspect ratio of most lesions is <1. Undifferentiated carcinomas are poorly differentiated and highly malignant, and are usually found with large tumor size, often with infiltration of peritumoral and peripheral structures, and often with enlarged lymph nodes. Imaging signs can only initially determine the type of thyroid cancer, and pathologic biopsy is still needed to diagnose the type of thyroid cancer. Patients are advised to follow the doctor's instructions for further examination and treatment according to the results of the imaging evaluation.