The left leg is thinner than the right leg, which may be caused by uneven walking, muscle atrophy and lack of exercise. 1. Uneven walking force: when walking activities, habitual use of the right side of the limb, will lead to the right side of the limb muscles than the left side of the limb muscle development, strength, which will appear in the left leg than the right leg of the situation. 2. Muscle atrophy: the left leg injury will cause muscle pain, which will affect the limb activities, affect the local blood circulation, which will lead to muscle atrophy of the left leg, so that the left leg is thinner than the right leg. 3. Lack of exercise: lack of exercise for a long time in the left leg will affect the blood circulation of the left leg muscles, so that the local muscle nutrition is insufficient and the symptoms of the left leg are smaller. If the difference between the thickness of the two legs is more obvious, it is recommended to go to the hospital in time to find out the cause of the disease.