The pleural effusion in b2 thymoma is considered to be caused by thymoma surgery, hypoproteinemia, compression of superior vena cava and other factors. 1. Thymoma surgery: when performing thymoma surgery, if the volume of thymoma is large, then the trauma will be larger, and it is very easy to cause pleural effusion after surgery. 2. Hypoproteinemia: malignant thymoma will cause malnutrition and hypoproteinemia, etc. When hypoproteinemia occurs, colloid osmotic pressure of pleural capillaries will be reduced, which will cause pleural effusion. 3. Compression of superior vena cava: when thymoma compresses superior vena cava, it will lead to the obstruction of venous return, causing the increase of vascular hydrostatic pressure, which will eventually cause pleural effusion. Patients with type b2 thymoma who have pleural effusion are recommended to go to hospital and ask the doctor to make judgment and give treatment plan, and do not use medicine by themselves.