Two- layer diffractive optics can achieve high diffraction efficiency at broad wavelength band. Manufacturing errors can result in loss of diffraction efficiency and further, influence modulation transfer function (MTF) of refractive and diffractive hybrid imaging optical system. Based on the expression of phase delay and polychromatic integral diffraction efficiency (PIDE), mathematical analysis model of relationship between manufacturing errors and polychromatic integral diffraction efficiency is presented. Two kinds of manufacturing errors applied in 8~12 μm far infrared optical system which results in diffraction efficiency of two-layer diffractive optics are studied. Method of PIDE can evaluate image quality of optical system over broadband wavelength. According to the relationship between PIDE and manufacturing errors, MTF for hybrid optical system consisting of two-layer diffractive optics can be analyzed.