Formerly known as malignant fibrous histiocytoma, pleomorphic undifferentiated sarcoma is a high-grade sarcoma without any specific direction of differentiation and is the most common type of soft tissue sarcoma. Pleomorphic undifferentiated sarcoma is a common soft tissue sarcoma that is highly malignant and recurrence and metastasis are common. The discussion on the tissue origin of pleomorphic undifferentiated sarcoma has never stopped and is still inconclusive, with the main doctrines being undifferentiated mesenchymal cell, fibroblast, and histiocyte. Numerous studies have shown that pleomorphic undifferentiated sarcoma is a type of sarcoma originating from fibroblasts or primitive mesenchymal cells, and is capable of biphasic differentiation to fibroblasts and histiocytes. Pleomorphic undifferentiated sarcoma is highly aggressive and has a poor prognosis due to the fact that the clinical manifestations are mostly painless masses and the tumor site is deep, and it is mostly found in advanced stages. If the above symptoms appear, go to the hospital for examination immediately to avoid delay of the disease.