The swelling and pain in the leg behind the knee is also known as popliteal swelling and pain, which can be caused by the following reasons: first, popliteal cysts, popliteal cysts can have swelling and pain in the popliteal fossa behind the knee when they grow larger, and cystic swellings can be palpated during the body check, which can have pressure and pain, and then the popliteal ultrasound and MRI should be checked for early diagnosis and early treatment. Second, bursitis of the popliteal fossa, when the bursa of the popliteal fossa has chronic inflammation, it can be accompanied by exudation, which leads to fluid accumulation in the bursa, resulting in distension and pain in the popliteal fossa, which should be examined by MRI of the popliteal fossa and physical examination for diagnosis. Third, soft tissue tumors behind the popliteal fossa, when the soft tissue behind the popliteal fossa has a tumor, can also have the symptoms of swelling and pain in the popliteal fossa behind the knee, when the tumor is relatively large can be touched on the surface of the body, this time to check the MRI and ultrasound to early diagnosis and treatment.