The bandwidth of s-polarization and p-polarization will separate when the narrow-band thin-film filter is in tilted incidence,which will limit the tunable range of the filter. The phenomenon of the polarization bandwidth separation and its relation to the mirror layers of the filter are analyzed. A method about eliminating the polarization bandwidth separation is also presented. According to this method,a former designed 100 GHz dense wavelength division multiplexing four-cavities angle-tuned narrow-band filter is improved. The simulation results prove it can eliminate the polarization bandwidth separation and enlarge the tunable range.