We have proposed a novel transmission slanted grating at the central wavelength of 1550 nm, which can be used in optical communication. We have presented an approximate analytical expression that provides an insightful physical description of the simplified modal method for the slanted grating. The odd grating mode, which only exists in the asymmetric structure under normal incidence, plays the positive role of enhancing the -1st order diffraction efficiency. The analytic expressions of mode conversion and coupling can be obtained to explain the asymmetric field distribution, which cannot occur in the rectangular grating region. Numerical results achieved by the rigorous wave analysis verify the validity of the simplified modal method. We expect that the theoretical modal method set forth in this work will be helpful for the tremendous potential application of the slanted grating.