Weakly modulated dielectric grating can be equivalent to planar waveguide. When diffracted high-order spatial sub-waves are coupled with the leak mode of the grating,guided-mode resonance occurs. In this paper,an analytical expression for the resonant position of weakly modulated dielectric grating is derived from the eigenvalue equation of planar waveguide. Its predicted results are consistent with the rigorous coupled-wave theory. The phenomenon that guided-mode resonance causes double reflection peaks at normal incident is explained for the first time,and the reasons that guide-modes is sensitive to the incident angle and the grating period are also discussed. In addition,the way to get reflection filters with good peculiarities is analyzed theoretically from the equation of characteristic matrix of the thin film