Layer uniformity was improved by using a wire mask in the fabrication of a coarse wavelength division multiplexer (CWDM) filter. Theoretical simulations determined the optimal diameter of the wire to be placed just below the substrate, which was rotated at 800 rpm during E-beam evaporation. This simulation also demonstrated the correction of the thickness distribution by the etching effect of ion-assisted deposition. In the corresponding experiment, a distribution uniformity of 0.083% in the radial range from 35 to 65 mm was achieved by the coating of the CWDM filter.