Pericardial effusion after cardiac surgery is mostly considered to be caused by post pericardiotomy syndrome, intraoperative localized pericardial injury, postoperative infection and viral infection. 1. Post-pericardiotomy syndrome: This disease mostly occurs in patients who have undergone open pericardial surgery, especially in patients with mitral valve surgery for rheumatic heart disease, and is considered to be a recurrence of rheumatic fever. It is characterized by clinical syndromes such as pericardial effusion, fever, and pleurisy. 2. Intraoperative local pericardial injury: If improper operation produces damage to the local pericardium during cardiac surgery, pericardial effusion after cardiac surgery is likely to occur. 3. Postoperative bacterial infection: If bacterial infection occurs after or during cardiac surgery, it induces pericarditis and causes fluid retention in the pericardial cavity, thus resulting in pericardial effusion after cardiac surgery. 4. Viral infection: the most common cause of pericardial effusion is viral infection, and the common viruses are Coxsackie virus A and B, and hepatitis virus. After surgery, the body produces a stressful process, immunity decreases, viral infection induces myocarditis and produces pericardial effusion. If pericardial effusion occurs after surgery, it is recommended to have a timely examination, diagnosis and treatment under the guidance of a physician, so as not to miss the condition.