The overall efficiency and image uniformity are important criteria for augmented reality display. The conventional in-coupling grating design intending to improve only the first-order diffraction efficiency without considering the multiple interactions with diffracted light in the waveguide is insufficient. In this work, the back-coupling loss (BCL) on the in-coupling surface relief grating, and the power of light arriving at the out-coupling grating over that of incident light (denoted as optical efficiency in waveguide, OEW) are introduced for the design of in-coupling grating. A simple and effective method to increase diffraction efficiency with unique angular selectivity is demonstrated by inserting an interlayer between the waveguide and grating. The optimized average OEW and its uniformity under a field of view of 40° are increased from 8.02% and 24.83% to 8.34% and 35.02% by introducing a region-selective MgF₂ interlayer.