The field of view (FOV) of the traditional Cassegrain optical system is small, generally only 0.1°—0.2°. In order to enlarge the FOV, it is usually necessary to add correction groups or change the mirror type, but it introduces color difference and complicates the structure. In this study, an optical digital joint design method is put forward to expand the FOV of the Cassegrain optical system. First, the structural parameters of the system are optimized to control the aberration. Then, based on the wavefront aberration theory, the wavefront aberration model is constructed using Zernike polynomials, and the point spread function model is established using Fourier transform. Finally, the image is processed using the spatial transform deconvolution algorithm. The FOV of the Cassegrain optical system is expanded using only primary and secondary mirror structures. The simulation experiment of the Cassegrain optical system shows that the FOV is expanded approximately 6 times, and the imaging quality is improved. The simulation results indicate that our method is feasible.