Pain below the knee can be caused by the following: first, patellar tendonitis of the knee joint, when the patellar tendon has chronic inflammation, it can lead to pain at the patellar tendon below the knee, which should be diagnosed and treated by early examination, MRI of the patellar tendon and X-rays and CT of the knee joint. Second, chondromalacia of the tibial tuberosity. Chondromalacia of the tibial tuberosity is common in adolescents, when the tibial tuberosity is not fully developed and can be damaged after a strenuous exercise, leading to tibial tuberosity. At this time, it is important to conduct a physical examination and check the x-ray and CT of the knee joint to make a diagnosis. Third, damage to the infrapatellar fat pad. When there is damage to the infrapatellar fat pad of the knee joint, there can also be pain below the knee, and the patient should have an early examination and MRI of the knee joint for diagnosis.