Left lumbar swelling can be caused by the following reasons: first, kidney disease, when the kidney has a tumor or inflammation, can have the symptoms of one side of the lumbar swelling, can be accompanied by slight hematuria and proteinuria, this time to check the kidney ultrasound, CT, imaging and urine tests for early diagnosis and early treatment. Second, lumbar muscle strain, when one side of the lumbar muscle has chronic strain, can appear the symptoms of lumbar swelling, often after cold or exertion, the symptoms of swelling are more serious, then we should check the lumbar spine X-ray, CT and physical examination for early diagnosis and early treatment. Thirdly, tumor in one side of the lumbar muscle, if the tumor is large, it can have the symptoms of lumbar distension and pain, and it can also have the symptoms of soreness and swelling, so it is necessary to check the ultrasound and MRI of the muscle at an early stage for diagnosis and treatment.