There are several possible causes of knee pain when bending: First, osteoarthritis of the knee. Severe osteoarthritis of the knee can involve the soft tissues and cartilage behind the knee, resulting in pain when bending the knee, which should be examined by knee X-ray and CT for early diagnosis and treatment. Second, patellofemoral arthritis, patellofemoral arthritis can also occur when the knee bends between the patella and the femur increased friction, resulting in pain, then we should check the knee X-ray, nuclear magnetic, to early diagnosis and treatment. Third, meniscal injury, when there is a tear or injury to the posterior horn of the meniscus, there can be pain when bending the knee joint, which can be accompanied by popping and interlocking, and early MRI of the knee joint should be taken for early diagnosis and treatment. Fourth, popliteal fossa disease, when there is popliteal cyst or bursitis, can also appear pain when bending the knee joint, early ultrasound and MRI of the popliteal fossa to diagnose.