The pain on the left side behind the waist may be caused by lumbar dorsal myofasciitis, urinary system diseases, acute lumbar sprain, and transverse fracture of the left lumbar vertebrae. 1. Lumbar dorsal fasciitis: due to repeated bending, prolonged sitting and standing or prolonged heavy physical labor, resulting in the emergence of aseptic inflammation of the lumbar dorsal muscles, which will lead to the patient’s lower back pain and discomfort and activity restriction and other symptoms. 2. Urological diseases: such as urinary stones, chronic nephritis, pyelonephritis, etc., which may cause back pain; 3. Acute lumbar sprain: it can cause the attachment points of lumbosacral muscles, as well as fascia and ligaments and other tissues to be torn. In addition, it may also cause the patient to experience low back pain and be unable to move normally. 4. Left lumbar transverse process fracture: left lumbar transverse process fracture can also lead to pain in the left side of the back. There are many other causes of pain in the left side behind the waist, you need to consult the doctor in time, under the guidance of the doctor for examination and treatment, avoid self-medication, in order to avoid adverse effects.