Sudden swelling of the knee joint can be caused by the following reasons: First, gout, which can involve the knee joint and lead to secondary synovitis and sudden swelling of the knee joint, which should be diagnosed early based on the patient’s medical history, blood uric acid tests and MRI. Secondly, rheumatism and rheumatoid rheumatism can also cause sudden swelling of the knee joint, which can be accompanied by joint pain and limited movement, and early diagnosis should be made by blood tests and X-ray examination of the knee joint. Third, synovitis caused by cold or strain can also cause swelling of the knee joint, so early blood tests and MRI and CT examinations of the knee joint should be performed for early diagnosis and treatment.