For the subwavelength grating guided mode resonance filter (GMRF) with a spacer layer, the drift mechanism of reflective phase was analyzed, and the relation between the optical wave phase and the spectral linewidth was investigated with the planar waveguide theory and the principle of guided mode resonance. On this basis, the super narrow linewidth resonator with the wavelength of 852 nm is designed. The simulation results show that the larger slope of phase as a function of wavelength (dφ/dΛ) becomes in the resonant range, the narrower linewidth (Δλ) is shown in the reflection spectrum. Furthermore, the energy of electric field concentrated in the waveguide layer is also higher in the corresponding field distribution. Different structural parameters have different effects on reflection characteristics. As the thickness of the grating layer decreases, the slope of the phase (dφ/dλ) increases from 1.0419 to 12.2666, and the linewidth decreases by 90%. With the increase of spacer layer thickness , the phase slope (dφ/dλ) increases from 2.04 to 7.15, and then the Q (Q≈λ0/Δλ) value goes from 1058.382 to 5174.393. The variation of the period of the subwavelength grating mainly affects the location of the resonance center wavelength, but has little effect on the rate of phase swing. By optimizing these parameters, the coupled resonant cavity with a narrow linewidth 0.002nm is designed, and the Q reaches 4.26×105 .