There is a contradiction between high resolution and light weight of large aperture spaced imaging system. So many research institutions have begun to explore new imaging methods. Diffractive lens through microstructure to modu-late light waves can be fabricated on thin films with very low surface mass density. With the benefits of small size, light weight and loose surface tolerance, diffraction imaging system has become a great potential technical solution. Fresnel zone plate (FZP) and photon sieves are the most commonly used microstructures, while the low diffraction efficiency limits the applications of photon sieves. FZP fabricated by binary optics technology can achieve 40.5% diffraction efficiency when the level is 2, and 81% for 4 levels. On the other hand, the diffraction efficiency is a function of the ratio of the design wavelength to the illumination wavelength. Therefore, the non design orders dif-fractive light may affect the performance of diffractive imaging system and can’t be ignored. In order to study the effect of diffractive stray light caused by non design orders on the modulation transfer function of diffractive imag-ing system, the wave propagation method was used to simulate the propagation of diffractive light waves. By co-herent superposition of finite diffractive orders, we calculated the PSF at 17 signal wavelength which evenly covers the spectral range. The sum of these results is the polychromatic PSF. After the point spread function (PSF) of the system was obtained, the Fourier transform of the PSF was done to calculate the modulation transfer function (MTF). The differences between the modified values and the theoretical design values were analyzed when the number of level was 2, 4 and 8 at the diffraction imaging system with an 80 mm Fresnel lens as primary lens. The MTF decreased at low frequency with 2-level Fresnel primary lens and the biggest decrease was 6.6%. The deviation from the design value is less than 0.5% when the level is 4 and 8. The results show that the effect of diffractive stray light on the MTF of the system decreases with the increase of the number of level. Finally, we found that only the incident light illuminating the primary’s central area can directly attach the image plane by non design diffractive orders. So, we put forward the idea that the central area of the diffractive primary lens is processed into 4 or 8 levels and the edge part is 2 levels to reduce the effect of diffractive stray light. The MTF increased apparently after opti-mized and was close to the design value. It shows that the idea can achieve the goal of suppressing the diffractive stray light.