An optical system is provided and includes a light-sensing element, a reflecting unit and a first driving assembly. The reflecting unit includes a reflecting surface, configured to receive an incident light and to reflect a reflecting light. The reflecting light travels into the light-sensing element. The first driving assembly is configured to control the reflecting unit to move along a first axis direction from a first position to a second position, so as to adjust a focus position of the reflecting light on the light-sensing element, wherein the reflecting surface in the first position is parallel to the reflecting surface in the second position, and a distance between a center of the reflecting unit in the first position and the center of the reflecting unit in the second position is not zero.