A modification of simplified modal method (MSMM) is presented to calculate the transmission efficiencies of subwavelength fused-silica grating. In order to consider the reflection of grating, which is neglected in simplified modal method (SMM). Then the grating region is divided into N thin planer grating slabs and regarded as multilayer films. Then the reflectance is characterized by the effective admittances of air and grating which are complete characteristic of multilayer films. This proposes a manner of reducing the reflection loss of dielectric grating. By analyzing the effective index and admittance of grating, we designed a triangular fused-silica grating with more than 99.9% diffraction efficiency for both T E and T M -polarization. This method offers a clear physical insight of diffraction characteristic: a deep-etched grating would get a high efficiency when the average difference of its mode indices is odd times of π and the admittance of the grating is approximate to that of air. Due to tiny deviation between this method and RCWA, MSMM can be used to design and optimize fused-silica gratings directly. It is better than SMM which only gives starting value of grating parameters. Contrast to RCWA, the starting parameters values of subwavelength dielectric grating can be predicted by the formulas in MSMM when the transmission efficiency is known. However, the starting values are got by cut-and-try procedure in RCWA.