As a result, the patient was generally doing well after surgery, and everything was handled as usual after cardiac surgery, and recovery was smooth. However, pericardial and pleural effusion started to appear on the 10th postoperative day, and pericardiocentesis and pericardial tube drainage were performed, and the pericardial tube was removed after 5 days. Bilateral thoracentesis was performed on an average of 2 to 3 days postoperatively, and an average of 1000 ml of yellowish fluid was withdrawn each time. Closed drainage of the left side of the chest was performed at 20 days postoperatively, and an average of 800 ml of yellowish fluid was drained daily. The fluid contained a large number of fat globules, which was suspected to be celiac disease, and the patient was treated with less fatty and oily food. Routine examination of pleural fluid and pathological examination of pericardial pleural fluid were performed respectively, and the results were as follows: pleural fluid had a large number of cells, especially leukocytes, and the lufanta test was positive. Pericardial fluid pathology showed lymphatic mononuclear-like cells and more erythrocytes, and no cancer cells were seen; pleural fluid pathology showed a large number of lymphatic mononuclear-like cells, erythrocytes and large nuclear deep-stained cells, and highly suspicious of cancer cells. Intraoperative snapshot and postoperative routine pathological specimens both suggested mucinous tumor with necrosis. The patient was discharged automatically with chest tube drainage and still had pleural effusion at three months postoperative follow-up. Discussion Primary cardiac tumors are relatively uncommon and are about one sixteenth of secondary cardiac tumors. Primary cardiac tumors are divided into two categories: benign and malignant, of which benign accounts for three-fourths and malignant accounts for one-fourth. Among the benign ones, cardiac mucinous tumor is the most common, followed by lipoma, teratoma, capillary hemangioma, lymphadenoma, lymphatic cyst, diverticulum, nodular granuloma, etc. Among the malignant ones, angiosarcoma is the most common, followed by mesenchymal sarcoma, mucosarcoma, mesenchymal cell tumor, etc. Cardiac mucinous neoplasm has no significant difference in the sex ratio between men and women, and it can develop in all age groups. Cardiac mucinous tumors are true primary tumors that originate from subendocardial or vascular retained cells or multiple latent retained cells, and are benign. Mucinous tumors are round or oval in shape, with varying depth of cut and lobulation. Cardiac mucinous tumors have a high surgical safety, high resection rate, and high cure rate. This patient was a primary cardiac tumor because other parts of the body where the tumor might have been located were ruled out preoperatively. And it originated from the junction of the right atrium at the superior vena cava, so it was of endocardial and vascular cellular origin. From preoperative echocardiography and cardiac MRI, intraoperative visual observation, postoperative freezing and conventional pathology all suggest mucinous tumor, so this case is a right atrial mucinous tumor, which is a primary benign cardiac tumor. However, postoperative pericardial and pleural effusions, especially pleural effusions, and pathology routinely found highly suspicious cancer cells, so cardiac malignant tumor could not be completely excluded again, which is the special feature of this case. The patient was treated with neck swelling and jugular vein thrombosis, while its cause was a mucinous tumor of the right atrium. The superior vena cava is not usually thrombosed, and this case is another typical presentation of superior vena cava syndrome. Superior vena cava syndrome is caused by significant narrowing and occlusion of the superior vena cava and both cephalic and brachial veins, resulting in obstruction of venous blood flow and swelling and bruising of the face, neck, and upper extremities. The upper superficial veins of the torso are angered and the venous pressure is increased. Common causes include intrathoracic malignant tumors, bronchial carcinoma, especially right upper lung lobe tumors, primary mediastinal tumors, lymphoma, and metastatic tumors. Less common are, mediastinal sarcoidosis, mycobacterial infection, tuberculosis, aortic aneurysm, idiopathic mediastinal fibrosis. The cause of this case is, however, a right atrial mucinous tumor, which is another special feature of this case due to obstruction of the superior vena cava into the right atrial port, causing thrombosis or occlusion of veins such as the innominate vein and jugular vein.