A 90° mixer plays an important role in achieving a high sensitivity detection in space coherent laser communication. The relationship between signal light polarization state and mixing efficiency of space coherent detection is studied. The circularly polarized light is more stable than the linearly polarized light used in traditional programs, and it is beneficial to achieving a high efficient mixing. A method that a 1/4 wave plate is introduced at the signal input end is proposed to change the polarization state of the signal light, and the simulation for the change of signal light polarization state is carried out when the signal light is influenced by external factors. The calculated signal power is used as a parameter, and the influence of the signal light polarization state on the mixing efficiency is studied. When incident local oscillator lights are same and other limited conditions are ignored, we come to a conclusion that the maximum relative signal power can be obtained by the mixer when the mixer receives circularly polarized signal light, and its mixing efficiency is the highest.