An optical system includes a fixed module, a movable module and a driving assembly. The movable module moves relative to the fixed module, and the movable module includes a lens unit which includes a first lens, a second lens, a first side wall and a second side wall. The first side wall has a first surface, which directly contacts the second lens, and the second side wall directly contacts the first lens. A portion of the driving assembly is directly disposed on the lens unit, configured to drive the lens unit to move along an optical axis of the first lens. The first side wall further has a second surface opposite to the first surface, and the second surface directly contacts the portion of the driving assembly. The thickness of the first side wall is different from the thickness of the second side wall.