Based on the theory of one-dimensional photonic crystal (1DPC), a flat-top polarization filter composed of cascaded thin-film glass cavities is proposed. A detailed design example is also given. The value of transmittance of P polarized light at transmittance window is higher than 99.6% and the maximum value of transmittance of S polarized light is about 0.1%. The center-frequency of transmittance windows for P polarized light accords with the ITU standard. On this basis, the influence of structural parameters of each reflecting film (including glass cavity) and incident angle on spectral performance such as rectangle degree, duty cycle and frequency spacing are investigated systematically. The proposed filter structure not only is very simple and easy to accomplish but also has high transmittance for P polarized light and good cut-off characteristic for S polarized light. It will have some applications in wavelength division multiplexing systems.