Swelling of the left side of the lumbar region is caused by a variety of reasons, such as lumbar trauma, acute left lumbar sprain, lumbar tumors and so on. 1. Trauma: bone or soft tissue injury in the left lumbar region due to external impact, manifested as obvious swelling of the left psoas major muscle, accompanied by symptoms of lumbar pain. 2. Acute left lumbar sprain: due to strenuous exercise, lifting heavy objects or excessive pulling and pulling caused by lumbar muscles, ligaments and other soft tissue injuries, manifested as obvious swelling of the left lumbar region, pain, and activity limitation. 3. Lumbar tumor: the left lumbar region will have pain and swelling due to the increasing of occupying lesions. There may be other reasons for the swelling of the left side of the waist, such as symptoms suggest timely medical treatment, improve the examination to clarify the cause of the disease, give targeted treatment or treatment.