The common causes of bruising in the knee joint are as follows: first, caused by injury, including strains and lacerations of ligaments caused by sports or accidental injury, as well as direct bumping and impact on the knee joint, resulting in vascular bleeding, blood spreading under the skin, and bruising and petechiae symptoms can be observed after the skin. Second, consider the factors of the blood system itself, such as abnormal platelet function, which causes abnormal coagulation mechanism, thus leading to local bleeding and producing bruising symptoms. In patients with hemophilia, the abnormal function of clotting factors, mainly factor III and factor VIII, can lead to disorders of the clotting mechanism, resulting in spontaneous bleeding and bruising. Third, nutritional aspects, such as the patient’s lack of vitamin K, can easily lead to abnormalities in the clotting mechanism, resulting in bruising of the knee joint. Patients are advised to observe and protect the knee joint if it is due to trauma, if the symptoms are not frequent after treatment, and if the bleeding bruises are not spontaneous. If the bruising occurs without external force and is frequent, it is important to seek medical attention.