Left knee pain on the right side is mainly caused by osteoarthritis of the knee joint, meniscus and ligament injuries, osteochondritis dissecans of the tibial tuberosity, chondromalacia of the patella, as well as autoimmune diseases and other etiologies. 1. Osteoarthritis of the knee joint occurs mostly in the elderly or patients with trauma to the knee joint, with pain aggravated by weight bearing and localized osteosclerosis and hyperplasia on X-ray. 2. Meniscus and ligament injuries: generally have a history of knee trauma, and can be clearly diagnosed by MRI. 3. Osteochondritis dissecans of tibial tuberosity: it usually occurs in adolescents, with obvious pressure pain at the tibial tuberosity, and the pain mostly appears after strenuous exercise. 4. Chondromalacia patellae: mostly occurs in young female patients, pain occurs when kneeling or walking up stairs. 5. Autoimmune diseases such as ankylosing spondylitis, the development of which will lead to knee pain and discomfort in the advanced stage. The causes of right-sided pain in the left knee are complex and need to be clearly diagnosed in conjunction with medical history, physical examination and auxiliary tests.