A large bag under the knee is often caused by the following reasons: first, joint cavity fluid, because under the knee is the joint cavity, if there is more local fluid, it will form a lump, you can go to the hospital to check the ultrasound of the knee joint, etc. for clarification. If there is a large amount of fluid, a puncture can be performed to drain the fluid and laboratory tests are needed to determine the nature of the fluid. If it is caused by an infection, antibiotics should be given to treat it. Secondly, it may be caused by sebaceous cysts, which is an infectious disease and needs to be clarified in the hospital as soon as possible. The disease needs to be treated surgically to prevent localized redness and swelling and breakage. Thirdly, it may be a lipoma, which is generally soft in texture and can be clarified by ultrasound. This disease usually does not require surgery and can be observed regularly. Fourth, some patients have other swellings, such as fibroids.