Objective　To explore the diagnosis and treatment of poorly differentiated endocardial sarcoma. Method　The clinical data of a child with poorly differentiated endocardial sarcoma was retrospectively analyzed. Results　One-year-old girl was admitted for diarrhea, polypnea, cyanosis, and cough. Abnormal heart sound was found by auscultation. Leads Ⅱ, Ⅲ, and aVF of ECG showed high peaked P wave. The diagnosis of poorly differentiated endocardial sarcoma was confirmed by echocardiography and pathology after cardiac operation. Three months after discharge from the hospital, the patient suddenly came into coma and died. Conclusion　The diagnosis of poorly differentiated endocardial sarcoma is mainly based on clinical manifestations, echocardiography and pathology. Surgical resection is the first choice and chemotherapy and radiotherapy play a supporting role. However, there is no cure for it currently.