As key filtering elements, guided-mode resonance (GMR) gratings have wide applications in optical communication. However, the transmission spectrum of a conventional GMR grating shows Lorentzian-type response, which limits the further application of GMR gratings in high-performance optical communication systems. Cascaded GMR gratings have been presented to realize flat-top filtering response, but the entire device has a large size and the fabrication process is complicated. In addition, it is difficult to achieve narrow-band flat-top filtering response with only a single-layer compound GMR grating. Hence, a cascaded double-layer compound grating is proposed to solve the problem in this work. The output spectra are analyzed by a combination of rigorous coupled wave analysis and eigenmode analysis, and the results show that the central wavelength of the filter is 1549.9 nm, and the line width of the flat-top spectrum is 0.5 nm.