Painless enlargement of submandibular lymph nodes is commonly caused by the following reasons: reactive hyperplasia of lymph nodes in the non-acute stage, lymph node metastasis, diseases of the blood system, and tuberculosis of lymph nodes. 1. Reactive hyperplasia of lymph nodes: mostly related to foreign body stimulation, bacterial infection and other factors, the lymph nodes will be enlarged, and after the acute stage, usually there will be no pain. 2. Lymph node metastasis: if malignant tumor occurs, such as thyroid cancer, laryngeal cancer, etc., it may lead to metastasis of submandibular lymph nodes, and the lymph nodes will be enlarged, usually without pain. 3. Diseases of blood system: if lymphoma, leukemia and other diseases occur, there will be multiple lymph nodes enlarged without pain, so painless and enlarged lymph nodes under both jaws are also possible. 4. Lymph node tuberculosis: it is caused by infection with tuberculosis bacillus, which will lead to enlarged lymph nodes, often accompanied by low fever, lethargy and fatigue, but without pain. It is recommended that the patient go to the hospital, can improve ultrasound, biopsy and puncture and other tests, usually can clarify the cause, and then can be treated accordingly.