Pain at the left buttock and lumbar junction may have the following causes: first, supragluteal dermatomal neuritis, if chronic inflammation of the supragluteal dermatomal nerve occurs, pain at the buttock and lumbar junction can occur, and relevant investigations should be performed for diagnosis; second, gluteal myofasciitis, patients with gluteal myofasciitis can also have chronic aseptic inflammation of the fascia at the buttock and lumbar junction on one side, resulting in pain; third, Sacroiliac joint disease, when sacroiliac joint sacroiliac arthritis or dense osteoarthritis, can also appear the pain of the buttocks and lumbar junction, then the relevant X-ray, CT examination for diagnosis; fourth, rheumatoid disease or ankylosing spondylitis, can also lead to pain at the buttocks and lumbar junction sacroiliac joint, pain can develop upward, gradually involving the entire spine, often in the morning when There are symptoms of morning stiffness, when blood tests as well as X-rays and nuclear magnetic examinations should be performed for early diagnosis and treatment.