A specific wavelength selecting method for an optical input device includes the following steps. A light source for respectively emitting at least two light rays in different wavelength regions is first enabled. Next, the two light rays are projected to a working surface to obtain at least two corresponding reflected light rays. Then, optical signals of the reflected light rays are detected and compared to obtain a preferred optical result. Finally, a light ray in the most suitable wavelength region is selected and enabled according to the preferred optical result.