Very little joint fluid is present in the normal moving joints of the body, which perform their physiological functions. When the fluid in the joint cavity significantly exceeds the normal situation, it is called joint effusion. There are many reasons for joint effusion, mainly due to increased exudation of synovial membrane, which exceeds its own absorption capacity and causes fluid retention due to loss of balance; there are also traumatic causes that lead to fluid accumulation in the joints; common inflammatory effusion can be caused by non-infectious and infectious factors; non-infectious factors commonly include osteoarthrosis, traumatic synovitis, rheumatoid arthritis, allergic diseases Non-infectious factors include osteoarthrosis, traumatic synovitis, rheumatoid arthritis, allergic diseases, etc. Infectious factors include septic arthritis and tuberculosis. Trauma, tumors, and pigmented villous nodular synovitis also damage intra-articular structures that may cause blood pooling. In addition, other systemic diseases such as hematologic disorders and kidney disease may also cause joint effusion to occur. As the specific cause of joint effusion varies, the clinical manifestations may also vary. Therefore, after the occurrence of joint effusion, doctors need to make clinical diagnosis and differential diagnosis, which may be determined by taking relevant blood tests, joint effusion aspiration tests, X-rays, ultrasound, MRI and other imaging tests depending on the situation.