Guide-mode resonances of double-layer gratings are analyzed with the Fourier modal method and the relationship between spectral characteristics of resonant gratings such as peak diffraction efficiency, linewidth and side-band reflectance and depth, period and fill ratio of grating are illustrated in detail. It is shown that the diffraction efficiency of resonant gratings will be higher than 99.5 percent when the groove depth tolerance is in the range of 15 percent, and mean while the relative error of line width will reach to 7 and 60 percent respectively for the negative and positive conditions. Compared with the position of resonant wavelength, the line width of resonant grating filters is relatively sensitive to the error of groove depth and residual thickness of waveguide layer when they deviate from designed values. Moveover, the resonant wavelength and width of spectrum do vary as a function of period and fill ratio.