1. An automatic magnification adjusting method applied to an optical navigation device, the optical navigation device having an illumination channel, a reference channel and an imaging channel, the reference channel having at least one reference feature with a known parameter, the reference feature comprising a plurality of reference points, and the known parameter representing a distance between the reference points, the automatic magnification adjusting method comprising: driving the illumination channel and the reference channel to alternatively output a detecting beam and a reference beam; capturing a series of navigation images consisting of a detecting image formed by the detecting beam and a reference image formed by the reference beam via the imaging channel; analyzing the reference image to recognize the reference feature; calculating a compensating factor according to a detected parameter of the reference feature; and adjusting magnification of the detecting image by the compensating factor to output corresponding coordinate displacement.