A Cassegrain optical antenna with parabolic structure is designed. By detailed analysis of Cassegrain optical antenna in partial axis situation，the expression of the area of the receiving spot and the power decline curve are obtained for different deflection angles. The relations between the received antenna gain and the wavelength and deflection angles are discussed in partial axis situation，the curves of gain for partial axis and axis alignment are simulated. The simulation result indicates that the gain of the maximal partial axis decreases by 6.564dB compared with that of axis alignment. Experiments of the optical spot test and the antenna coupling efficiency test are carried out separately for axis alignment and certain partial axis situations. The results show that the antenna coupling efficiency under partial axis situation is 26.97% lower than that of axis alignment. This study will provide a theoretical foundation for the control system to realise perfect alignment of optical axis in inter-satellite optical communications.