The present invention provides an object detecting system, adapted to a motion-control device, including at least one projecting unit, at least one image sensing unit and a processor. The projecting unit projects a plurality of sets of structured light corresponding to a first scanning resolution with a first scanning frequency toward a first direction. The image sensing unit senses the reflected structured light. The processor obtains a plurality of three-dimensional images corresponding to the first direction according to the sensed structured light, and determines whether there is at least one object that will move into a safe region of the motion-control device according to the three-dimensional images. When the processor detects the object, the processor adjusts the first scanning frequency and/or a scanning region of the projecting unit or the first scanning resolution.