A swollen knee is known as swelling. Swelling of the knee but no pain may be caused by bleeding, fluid accumulation and benign tumors in the knee. 1. Bleeding: patients with hemophilia are prone to blood accumulation in the knee, but such patients do not have pain in the joints in the early stage, and the joints are only swollen, but not painful. 2. Fluid accumulation: If there is arthritis or degeneration of joints in old age, the joints will produce more fluid, forming intra-articular fluid accumulation, and the knee will be swollen without pain. 3. Benign tumors: benign tumors in the knee will grow gradually in the knee, such as tendon sheath giant cell tumor of the knee, synovial chondroma or choriocarpal nodular synovitis, in the absence of the formation of obvious arthritis, there is no pain, but only joint swelling. In addition, knee swelling but no pain can also be seen in other causes, it is recommended to go to the hospital in time to avoid delay.