The obscuration caused by the Gaussian distribution of the laser beam in the space optical communication will affect the transmission efficiency of the antenna seriously,which leads to a drop in the quality of communication. The system structure of the optical antenna and the influence on the antenna gain is analyzed,which is caused by the obscuration of the secondary mirror in view of the Cassegrain. According to the transformation and transmission characteristics of Gaussian beams through the optical system,the effect of the obscuration and incident beam waist on the light transmittance is analyzed when Cassegrain antenna is used,and a new way is proposed to improve the transfer efficiency of the optical antenna. The improved antenna system makes its transmission efficiency improve significantly.