A thick leg and a thin leg may be physiological, or it may be caused by vascular diseases of the lower limbs, muscle atrophy on one side, lymphatic fluid return obstacles and other pathological reasons. 1. Physiological: part of the population due to heredity or long-term exercise on one side of the leg, there can be a side of a thick leg and a thin leg, is physiological development. 2. Lower limb vascular disease: commonly seen in one side of the lower limb venous thrombosis, varicose veins or venous compression and other diseases, due to blood stasis, the patient can appear one leg thick and one leg thin. 3. Muscle atrophy on one side: due to lumbar spine disease, lower limb fracture and joint disease causing muscle atrophy on one side of the limb or long term fixation leading to muscle and other tissues atrophy, the patient may have a thick leg and a thin leg. 4. Lymphatic fluid reflux obstruction: due to malignant lymphoma and other diseases, lymphatic fluid reflux obstruction, accumulation in subcutaneous tissues, limb swelling, patients may have one thick leg and one thin leg. There may be other reasons for one leg thick and one leg thin, it is recommended to go to the hospital in time, improve the examination to clarify the cause of the disease, and then give targeted treatment or therapy.