The relation between bandwidth integrated average diffraction efficiency (BIADE) of multi-layer diffractive optical elements (MLDOE) and corresponding phase retardation expression is discussed, and the maximum BIADE and the corresponding design wavelengths are determined when the optical materials are selected, then the design of the BIADE of the MLDOE is realized exactly and maximumly. Based on the principle, to design a MLDOE with (PMMA) and polycarbonate as materials respectively, the maximum BIADE is 99.3% in visible waveband (0.4～0.7 μm), the corresponding design wavelengths are 0.435 μm and 0.598 μm, the heights of harmonic diffractive element (HDE) are 16.460 μm and 12.813 μm respectively, and the BIADE of the MLDOE designed with this method is 3.9% higher than that with the former method in which design wavelengths are 0.4 μm and 0.7 μm.