Fluid accumulation in the knee can be either a normal physiological phenomenon or a clinical symptom caused by synovitis, osteoarthritis and other diseases. 1. Physiological phenomenon: the normal knee joint also has a certain amount of fluid, produced by the synovial membrane secretion, about 5 ml, the main role is to lubricate the joint surface, the normal joint fluid is yellowish or colorless, viscosity is very high. 2. Synovitis: Trauma and rheumatic diseases can lead to synovitis. When the synovial membrane is stimulated, it is easy to lead to excessive secretion of synovial fluid, which cannot be absorbed and metabolized by the body in time, and will lead to the collection of synovial fluid in the joint cavity, thus resulting in the production of fluid in the knee. 3. Osteoarthritis: Osteoarthritis is usually accompanied by inflammation, osteophytes and synovial hyperplasia, which can stimulate the production of a large amount of joint fluid, resulting in the production of fluid in the knee. Patients with fluid in the knee should go to the hospital in time if they experience any discomfort and receive regular treatment under the guidance of a doctor.