Based on Kogelnik′s coupled-wave theory, diffraction efficiency of reflection volume holographic grating is analyzed for several parameters. We propose a method to broaden the diffraction angular bandwidth by superposing multiplex volume holographic gratings, each of which is oriented properly. Permised on the requirement of uniform diffraction, the diffraction process in multiplex volume gratings is analyzed to acquire the offset of Bragg angle between adjacent gratings, and the uniform-diffraction bandwidth versus multiple numbers and refractive index modulation are finally obtained. Numerical results show that diffraction angular bandwidth of multiplex volume holographic gratings increases linearly with the number of multiplex gratings and refractive index modulation depth. The angular bandwidth can be broadened markedly via increasing the number of superposed gratings with appropriate refractive index modulation depth.