There are many reasons for popliteal pain when bending the knee, and the common causes are related to muscle strain, joint capsule and ligament injury, meniscus injury, bone tumor and osteoarthritis. 1. Muscle strain: popliteal muscle strain at the back of popliteal fossa mostly occurs in people who exercise too much for a long period of time, and the pain in popliteal fossa when bending the knee will occur after the muscle strain. 2. Joint capsule and ligament injury: long-term pulling or external force can cause damage to the knee joint capsule and ligament, resulting in local tissue congestion, edema, popliteal pain when bending the knee. 3. Meniscus injury: excessive flexion and rotation of the knee joint in sports can easily cause meniscus injury, meniscus injury, especially the posterior horn of the meniscus, can cause popliteal pain when bending the knee. 4. Bone tumor: bone tumor or metastatic cancer of distal femur and proximal tibia can cause pain in popliteal fossa when bending the knee because it invades bone tissue. 5. Osteoarthritis: osteoarthritis is caused by degenerative changes of cartilage of knee joint, which leads to damage of cartilage and subchondral bone, and when bending the knee, it will press the subchondral bone and cause pain, and then it will cause pain in popliteal fossa when bending the knee. Bending the knee popliteal pain may also have other causes, such as popliteal cyst, popliteal bursitis, and so on, it is recommended to go to the hospital in a timely manner to improve the examination, to clarify the cause of the disease, under the guidance of the doctor to carry out targeted treatment or treatment.