Firstly, the coherent efficiency with mismatching angle is deduced theoretically.Secondly, the relationship between the optimal relative aperture and coherent efficiency and the relationship between coherent efficiency and mismatching angle are simulated.Finally, the influence on coherent efficiency and receiving field when local oscillator is out of focus is evaluated.The results show that the receiving field of coherent detection is inversely proportional to antenna aperture in ideal conditions, and the overall performance of receiving antenna when local oscillator is out of focus is no better than that of small-aperture antenna when the effective coherent efficiency is identical.The results can serve as a theoretical basis of matching design between coherent detection system and optoelectronics system, and provide guidance on the antenna design, adjustment and measurement of coherent detection.