Possible causes of a soft bump on top of the knee include trauma, subcutaneous lipoma, meniscus cyst, etc. 1. Trauma: if due to trauma around the knee and other reasons, it will cause the soft tissue of the knee to swell, edema, congestion, etc., which is easy to form a localized bulging soft bag lump form. 2. Subcutaneous lipoma: It is a kind of movable mass protruding from the body surface with certain toughness. If it grows at the knee, the above symptoms may appear. 3. Meniscus cyst: it is a cystic change that occurs in the meniscus of the knee, which is usually seen in people with meniscus injuries, and can appear as a cystic bulge on the body surface. In addition to the above reasons, adenoma in the knee area and other diseases can also cause the above symptoms. It is recommended to go to the hospital in time, complete the examination to clarify the cause of the disease, and then follow the doctor’s instructions to give targeted treatment.