Sudden pain in the knee can be caused by several reasons: First, gout, gout involving the knee joint can appear sudden pain in the knee, often accompanied by local skin redness and swelling, serious cases may have gout stone formation, this can be early blood tests and knee x-ray examination, to early diagnosis and treatment. Second, patellofemoral arthritis, patellofemoral arthritis patients can appear at the knee pain, then we should combine the medical history and physical examination, early knee x-ray, nuclear magnetic examination to diagnose. Third, patellar tendonitis or quadriceps tendonitis is common in patients after a lot of sports and can present with pain above and below the patella, which should be diagnosed by early MRI and physical examination of the knee. Fourth, adolescents often have sudden pain due to cartilage damage to the tibial tuberosity, often after running and jumping or playing sports, and early diagnosis is made by taking x-rays and examining the knee.