Traditional off-axis reflecting optical antennas have the disadvantage of short exit pupil distance, leading to the fact that a relay optics is required to re-image the exit pupil on the fast steering mirror and thus the following optical paths and structures are complicate. Moreover, the backward scattering light in the relay optical system has a severe influence on the laser communication terminals. To overcome the above defects, we design an off-axis four-mirror optical antenna, in which the exit pupil of optical antenna is directly designed on the fast steering mirror and thus the relay optical system can be removed. First, the aberration formulas and the initial structural equation are deduced based on the theory of coaxial three-mirror aberration. Then, the initial structural parameters of the off-axis four-mirror optical antenna are calculated according to the aberration theory and the requirement of the exit pupil position. In addition, the ray tracing and the performance optimization of the initial structure are implemented using the Zemax software. Finally, the tolerance analysis of the off-axis four-mirror optical antenna is conducted. The results illustrate that the designed system has the advantages of excellent performance, compact structure, and reasonable manufacture and alignment tolerance. The performance of the designed system can meet the requirements of laser communication terminals.