Left back numbness is mainly considered to be related to neurological lesions, thoracic vertebrae small joint disorders, local blood circulation is not smooth. 1. Neurological lesions: when there are abnormal lesions of the nervous system in the thoracic area, such as spinal cord injury, etc., it is easy to affect the function of the nerves, resulting in a feeling of numbness in the left back. 2. Thoracic spine small joint disorder: usually, if the movement is not proper or the posture is bad for a long time, the thoracic spine small joint disorder will easily lead to the peripheral nerve compression, which will trigger the symptom of numbness in the left back, and even produce the performance of thoracic spine pain. 3. Local blood circulation is not unobstructed: the location of the left back, if a long time cold or usually overworked, easy to affect the local blood circulation, when the local blood circulation is not unobstructed, easily triggered by the left back of the numbness and crispness of the symptom, generally appropriate hot compresses, the symptom will be relieved. If the symptoms persist for a longer period of time, you need to check the specific cause of the disease, under the guidance of a professional physician targeted treatment, to prevent the later impact on the function.