Integrated optical filters based on guided-mode resonant gratings have potential applications in optical fiber communications. However, the output spectrum of a single guided-mode resonant grating is generally Lorentzian, which hinders the use of this type of structure in wavelength division multiplexing systems. Traditional methods generally use multiple resonator cascades to achieve a flat-top filter response. However, the entire structure is considerably large and the manufacturing process is complicated. Therefore, based on the output spectral response, a cascaded double-layer guided-mode resonant gratings structure of the flat-top and steep-edge type is proposed in this study. First, the physical model of the device is established. The physical mechanism is to combine the guided mode resonance effect with the Fabry-Perot resonance effect, and then use the physical model to analyze and design the filter structure. The center wavelength of the filter is observed to be 1550 nm, and its 3 dB bandwidth can be increased to 20 nm.