In order to improve the installation and adjustment efficiency of off-axis three-mirror optical antenna of space laser communication, it is necessary to optimize the design of total freedom of the mirror. Based on the principle of coaxial three-mirror afocal system, a method of integration of primary-tertiary mirror was proposed. The relationship between structural parameters of optical system was deduced, and an off-axis three-mirror optical antenna with compact structure and primary-tertiary mirror integration was designed by Zemax optical software. The design results show that the optical image quality of full field of view is better than diffraction limit, the primary mirror and tertiary mirror are close to each other in space, the common mother plate can be processed and surface shape can be detected by the common reference, which provides a method for integrated processing of primary mirror and three-mirror. The degree of installation and adjustment freedom of optical antenna is reduced by 6, which reduces the difficulty and improves the efficiency.