The pain in the posterior fossa of the knee, also known as the popliteal fossa, may have the following causes: first, injury to the meniscus, when the posterior horn of the meniscus is damaged, there can be pain on the posterior side of the knee, especially when squatting or flexing the knee joint. Second, popliteal cysts, popliteal cysts can appear in the popliteal fossa pain, often distension, can be felt in the popliteal fossa cystic swelling, then the relevant examination and nuclear magnetic, ultrasound examination to make a diagnosis. Third, bursitis in the popliteal fossa, there is a bursa in the popliteal fossa, when the bursa has a chronic aseptic inflammation, you can have pain in the popliteal fossa. Fourth, osteoarthritis of the knee joint, when osteoarthritis of the knee joint involves the soft tissues around the knee joint, pain in the popliteal fossa behind the knee can also occur, then the relevant x-ray examination should be performed for diagnosis and treatment.