In this paper, the energy loss caused by the secondary mirror of Cassegrain antenna has been discussed numerically. As the diameter of the secondary mirror is one-fifth of the primary mirror, the loss goes beyond 18.5%. Because of the alignment of the optical system being the significant factor, it influences the transmission quality of the optical antenna system. We analyze and numerically simulate how power attenuation ratio and gain of Cassegrain antenna vary with the antenna deflection, and measure the distance such that the central axis of the receiving antenna deviates from the z-axis in case of occurrence of off-axis space in the system. Finally, an optimum structure is proposed by trepanning the secondary mirror.