The tilt error introduced in the system assembling process affects the diffraction efficiency of diffractive optical elements (DOEs). An increase in the incident angle also affects the diffraction efficiency of DOEs. Based on the phase delay expression of multilayer DOEs (MLDOEs), this paper proposes a theoretical model of the relationship between the diffraction efficiency/bandwidth integrated average diffraction efficiency (BIADE) and the tilt error. The effect of the tilt error on the diffraction efficiency/BIADE at oblique incidence is analyzed. The relationship between the comprehensive BIADE working within a certain incidence angle and the tilt error is established. When the range of incident angle for the MLDOEs, operating within the 8--12-μm infrared waveband, is 0°--20°, if the comprehensive BIADE is required to be higher than 98%, the tilt error should be less than 0.25°. When there are other errors, such as the decenter error and the microstructure height error, the tilt error is further analyzed to reach the required comprehensive BIADE. The method and conclusions can be used to assist in designing and assembling the MLDOEs in hybrid optical systems.