Guided-mode resonance (GMR) reflection filter is presented by adding a buffer layer which is used in design of homogeneous thin-film reflection filters. We use the rigorous coupled-wave theory for studying the improvement of buffer layer to cut-off properties of reflection spectra. For the double layers structures under a TE-polarized plane wave illumination, the width of reflection spectra is broadened obviously from 192.4 nm to 345.6 nm by adding a buffer layer 97.5 nm, and the reflected sideband depressed more than the structure without buffer layer in the wavelength range of 650～1250 nm. Analogously,the similar structure under a TM-polarized plane wave illumination at the Brewster angle can obtain wider cut-off spectral range and lower reflectance than the one without buffer layer in the range of 700～1300 nm. By optimizing the structures and choosing the compatible parameters of grating, we can get better cut-off properties of reflection spectra and hold the line on the resonant wavelength.