The necessary and simple condition for obtaining high reflectivity at multiple frequencies has been further derived, and the general form of the multilayer system and the corresponding formula of controlling wavelength are given in the paper. To be an example, the computed transmittance curves for multiple-frequencies (2 ×, 3×, 4×, 5× ) are shown. We have experimentally studied the system of 2 x frequencies, on which the transmittance curves are presented.