Whether you can apply heat to a knee with effusion or not requires a specific judgment of the specific condition, and cannot be generalized. If it is caused by osteoarthritis or rheumatoid arthritis, patients can apply local heat to the knee joint to help recover from the condition. If the fluid or pus accumulation is caused by acute sports injury or intra-articular infectious disease, hot compresses should not be applied to avoid aggravating the condition. For acute sports injuries it is recommended that ice be applied first to constrict local blood vessels and relieve pain; hot compresses can be applied after 72 hours to promote the elimination of swelling. For infectious diseases, antibiotics should be administered intravenously under medical supervision and drainage should be performed under arthroscopy. Therefore, patients with fluid in the knee should not blindly apply hot compresses. It is recommended to go to the orthopedic department of the hospital for examination in a timely manner, to be judged by a professional physician, and then to take appropriate measures.