Knee pain when going down the stairs can be caused by the following reasons: First, patellofemoral arthritis of the knee joint, common in young people, due to long-term heavy exercise and damage to the patellofemoral cartilage, chronic inflammation, which can have increased friction when going up and down the stairs patellofemoral joint, resulting in joint pain, which should be checked early X-ray, MRI of the knee joint to diagnose and treat. Second, meniscal injury, when there is a tear in the posterior horn of the meniscus of the knee joint, there can be knee pain due to flexion of the knee joint when going down the stairs, resulting in meniscal extrusion, which should be diagnosed by early examination of MRI of the meniscus. Third, osteoarthritis of the knee joint, severe osteoarthritis of the knee joint can also have the symptoms of knee pain when going down the stairs, which should be diagnosed and treated by early examination of knee X-ray and CT.