A method for adjusting an optical system is provided, including a positioning device positioning a first optical module; a measuring device measuring an angular difference between a main axis of the first optical module and an optical axis of an optical element sustained by the first optical module to obtain a measurement information; an adjusting device changing the shape of an adjustment assembly of the first optical module according to the measurement information; and assembling the first optical module with an optical object, wherein the optical axis of the optical element is parallel to a central axis of the optical object.