Knee joint clicking, may be knee cartilage damage, meniscus injury, knee osteoarthritis, patella subluxation and other reasons. 1. Knee cartilage damage: knee joint trauma or long-term load sports, will cause knee cartilage damage or even fall off, resulting in cartilage surface roughness, which in turn causes the knee joint activities appear clicking abnormal sound. Knee MRI or arthroscopy can assist in the diagnosis. 2. Meniscus injury: knee meniscus injury or tear can cause poor alignment of the femoral condyle and meniscus, resulting in abnormal clicking sound or joint strangulation during knee flexion and extension activities. MRI or arthroscopy of the knee can assist in the diagnosis. 3. Knee osteoarthritis: Knee osteoarthritis will be caused by degenerative changes of meniscus and cartilage in the late stage, which will cause symptoms such as clicking or popping of the knee joint. X-ray examination and MRI of the knee joint can assist in the diagnosis. 4. Patellar subluxation: Patellar subluxation will appear popping and clicking when the knee joint extends and flexes, which is mainly caused by patellofemoral joint facing abnormality. Knee CT, MRI can assist in the diagnosis. Knee joint clicking, should actively consult the doctor, clear cause for targeted treatment.