Of the many joints in the human body, the knee joint has the most complex structure, the largest intra-articular space, and the most abundant synovial tissue. In addition, the knee joint is the joint that is used most frequently in almost all daily activities, and therefore has the greatest chance of synovial injury, which explains why synovitis is most often seen in the knee joint. Most synovitis of the knee is complicated by the various knee injuries mentioned above, but it can also develop alone or secondary to osteoarthritis of the knee, the latter mostly in the elderly. In young adults, there is a history of acute knee trauma, and mild edema, pain, limited motion, and claudication begin to occur in the knee after the injury. Synovial reactive effusion usually appears 6 – 8 hours after the injury, and the knee joint is significantly swollen, warm and afraid to move. Examination reveals limited knee flexion and extension, difficulty squatting with pain, and there may be limited pressure pain points around the joint with a positive floating patella test. Chronic injury to the synovium, which may not have a clear history of trauma, is characterized by tenderness and limitation of knee movement, swelling that does not subside, and fear of squatting. The swelling is persistent and prevents squatting. It increases with increased activity and decreases with rest. In patients with prolonged disease, hypertrophy of the knee capsule may be observed. For those who have a lot of fluid in the knee joint or those who have repeated fluid accumulation, joint fluid examination can be done, which can reflect the nature and severity of synovitis. Therefore, arthrocentesis and synovial fluid examination are important reference values for both the diagnosis and differential diagnosis of synovitis of the knee. The secret to treating synovitis is six words – rest, anti-inflammatory, and swelling reduction. Synovial damage is the starting point of the disease, so once synovitis is diagnosed, the first thing to do is to give the joint some rest to prevent the synovial membrane from being bruised again and inflammation from worsening; synovial inflammation is the course of the disease, so anti-inflammatory drugs should be used as early as possible and taken regularly to stop the synovial membrane from continuing to secrete inflammatory fluid to reduce the symptoms of pain and swelling. Joint swelling is a manifestation of the disease, so drugs that promote the absorption of joint fluid should be used, along with physical therapy to accelerate blood circulation around the joints and reduce swelling as early as possible. Early, standardized and reasonable drug treatment is the guideline for the treatment of synovitis. The natural course of synovitis is usually about 1-2 months. After regular treatment, the symptoms will be gradually relieved and the effusion will slowly disappear. Patients need to be patient, take their medications regularly and protect their joints in their daily life. If treatment is delayed, medication is inappropriate, or rest is inadequate, synovitis can last for months or longer, and long-term synovial inflammation can accelerate the aging of synovium and cartilage and increase the risk of infection in the joint cavity.