Pain on the side of the knee may be caused by osteoarthritis of the knee joint, soft tissue injury of the knee joint and so on. 1. Osteoarthritis of the knee joint: osteoarthritis of the knee joint, the pathogenesis of which is not clear at present, and it is believed to be related to degenerative diseases of the knee joint. Degeneration of the knee joint can lead to a decrease in the stability of the knee joint, and the emergence of bony encumbrances in the knee joint, which leads to an increase in wear and tear of the joints, and joint aging, and then pain in the lateral side of the knee occurs. 2. Knee joint soft tissue injury: the inner and outer side of the knee joint has meniscus and the inner and outer collateral ligaments and other soft tissues, sports injuries or trauma may cause damage to these soft tissues, resulting in structural abnormalities within the knee joint, affecting the normal movement of the joints, and the symptom may also occur. There may also be other reasons for the pain on the side of the knee, it is recommended to go to the hospital in time, improve the examination to clarify the cause of the disease, and then give targeted treatment or treatment under the guidance of the doctor.