According to technical requirements of satellite optical communication, a set of optical system of transmitter and receiver with a common optical antenna is designed at 850nm. We select a Cassegrain-type afocal off-axis system as the optical antenna structure. The entrance pupil diameter is 150 mm. The field of view of transmitter and receiver is ±100μrad and ±5mrad, respectively. This optical system has a simple and feasible configuration. The design results show that system performances are acceptable. The MTF is close to the diffraction limit. The energy concentration ratios are more than 90% at 30μm of diameter of circle. The RMS of wave front aberration is less than λ/20.