The coaxial two-mirror reflective structure has the disadvantage of small receiving field of view and low emission efficiency when being used as optical antenna in space laser communication. At the same time, the energy loss caused by central obscuration will greatly reduce the aiming and capturing probability of communication system. For this inherent defects, an off-axis two-mirror reflective structure with freeform surface was designed to improve the performance of space laser communication. The initial structure of the off-axis two-mirror reflective structure was deduced according to the basic aberration theory. Zernike polynomial freeform surface was introduced to improve the ability of designing freedom and balancing the freeform surfaces aberration. A design example was given whose coaxial and off-axis two-mirror reflective system had same pupil aperture and focal length, and the modulation transfer function, spot diagram, diffraction encircled energy and wavefront error were compared. The results show that the image quality of the off-axis two-mirror reflective antenna with freeform surface is better than that of the coaxial, thus, the structure of the off-axis two-mirror reflective antenna structure could improve entire performance of space laser communication system.