Based on the fact that Bragg reflection grating (BRG) is a special case of one-dimensional photonic crystal, we may apply the one-dimensional photonic crystal band gap theory to design the BRG. OptiFDTD is used to simulate both the width error and tilt angle error in BRG. It turns out that the diffraction efficiency will stay high at the 10% width error or the 10° tilt angle error of the Bragg grating. These indicate that the BRG has a high fabrication tolerance. One model of a Bragg diffraction double-grating in the Rowland circle structure is proposed based on the one-dimensional photonic crystal band gap theory for achieving the two-band spectral diffraction. Result shows that the diffraction efficiency of the two incident bands can be as high as 70% when the first set of Bragg grating periods is 6 layers while the second is 10. It is significantly higher than the traditional deeply etched diffraction grating. This is the foundation of a new type of EDG-WDMer with its advantages being of small size and high diffraction efficiency. It may have great potentiality for developing high diffraction efficiency dense wavelength division multiplexer in the future.