The common causes of bruising on the knee joint are as follows: 1. Frequent injury or strain on the knee joint, resulting in rupture of the local blood vessels in the knee joint, blood outflow and infiltration into the subcutaneous tissues, will be seen on the skin bruising symptoms, to avoid frequent injury and strain on the knee joint; 2. This is often accompanied by other symptoms of the whole body, such as fever or other immune reactions; 3, seen in hemophilia, due to abnormalities in the body clotting factors, insufficient activity of clotting factors or low content of clotting factors, resulting in abnormal blood clotting in the blood vessels when there is a slight bump or activity, which leads to bleeding and always bruising of the knee. Patients are advised to go to the hospital for detailed examination, and it is better to have timely treatment for this condition.