The structure of Cassegrain optical system severely limits its field of view (FOV). Generally, additional optical elements are needed to expand the FOV, but this approach leads to a complicated structure and does not facilitate a miniaturized and lightweight optical system. In this paper, a new method to expand the FOV of Cassegrain system using computational imaging was proposed. First, the structural parameters of the system were optimized to control the aberrations; then, a point spread function model was established; finally, a deconvolution algorithm was used to process the image. Eventually, the FOV of the Cassegrain system could be expanded using the structure that only comprised primary and secondary mirrors. A simulation experiment was conducted wherein a Cassegrain system with an F of 5.5 and 470-mm focal length was developed. The experimental result shows that in the FOV of 1.5°, the off-axis FOV modulation transfer function of the processed image is approximately improved by 0.2 at 20 lp/mm. Moreover, the quality of the image is significantly improved.