A fine-displacement adjusting device is constructed to have a first base portion and a second base portion spacedly encircled therewithin. The fine-displacement adjusting device further includes a first retractable element biased between the first and second base portions, and a first resilient element biased between the first and second base portions. The first retractable element is configured to extend and retract its length along a first axis to move the second base portion in a reciprocating manner. When the second base portion is moved by the first retractable element, the first resilient element moves the second base portion along the first axis to restore the second base portion. Therefore, the second base portion is fine-moved along the first axis in a reciprocating manner for adjusting the fine-displacement of an optical lens relative to an optical light source of a light engine in the first axis.