A separated guided-mode resonance filter was presented, which is consisted of a grating layer and two planar dielectric waveguide layers partitioned by an air gap. The optical responses of the grating under different parameters are analyzed by time domain finite difference method. The research shows that the resonance wavelength of the grating can be tuned by varying the height of air gap when the TM polarization incident wave is applied.Furthermore, the resonance wavelengths almost vary in a linear relation respect to the height of air gap. A narrow linewidth characteristic can be obtained by applying a shallow grating. The simulation results show that the wavelength can be tuned from 1 515 to 1 558 nm with the FWHM less than 0.6 nm.