After the left side of the back pain may be lumbar muscle strain, the left side of the ureteral stones, lumbar disc herniation and other reasons. 1. Lumbar muscle strain: long-term sitting position inactivity, or long-term stooping to carry heavy loads and other physical labor, easy to lead to spasm and fatigue of the left lumbar muscles, resulting in muscle strain or traction injury, may appear left back side lumbar pain symptoms. 2. Left ureteral stone: when the stone moves in the left ureter, it can be manifested as severe pain around the waist and may be accompanied by symptoms such as hematuria. 3. Lumbar disc herniation: the lumbar spine will have different degrees of degenerative changes, when the herniated disc compresses the nerve root, it can cause pain in the corresponding part of the nerve, radioactive pain in the lumbar region, left buttock, left lower limb, as well as numbness in the lower limb. Posterior left-sided back pain may also be caused by other reasons, it is recommended that the patient seek medical attention in a timely manner to clarify the cause of the disease, under the guidance of the doctor for targeted treatment.