A wavelength selective switch (WSS) is the core device of the current- and next-generation reconfigurable optical add-drop multiplexer and dynamic all-optical network, which is used to provide high port counts and spectral modulation fitness. In this study, we simulate a WSS system using the ZEMAX optical software. Further, a Fourier lens group and double cylinder lens are employed to improve the optical structure of WSS. The WSS optical structure is designed and optimized by using a merit function, a spot diagram, and an optical transfer function. Subsequently, the imaging capability of the entire WSS optical system is gradually improved. Finally,a 65-port WSS optical system with a minimum spectral filtering bandwidth of 8 GHz is obtained.