A design method of the computational flat diffractive computational flat diffractive optical system is presented to simplify the optical system structure and achieve high image quality. The aberration expression of the flat diffractive optical element (FDOE) is derived, and then computational imaging methods are used to eliminate the influence of off-axis aberration on image quality, so the field of view is expanded. Based on theoretical analysis, the FDOE is designed, and the field of view has been expanded from 2° to 5°. The results show that the detail resolution of the edge field of view is enhanced after restoration, and the modulation transfer function (MTF) of different subareas calculated using the slanted-edge method improved by an average of 0.17. The diffraction efficiency of the FDOE is greater than 95.75%. This method realizes the miniaturization and lightweight of the optical system, and provides new ideas for the integration of optical systems.