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When coherent detection occurs, the polarization mismatch between signal light and local oscillator light can reduce the efficiency of coherent detection. This article combines the principle of optical mixers to derive the relationship between the polarization state and mixing efficiency of signal light and local oscillator light, and builds an experimental platform for the coherent detection of a partially coherent electromagnetic Gaussian Schell beam (EGSM). Polarization devices are used to regulate the polarization state of the signal EGSM light and local oscillator EGSM light, and different polarization states of the EGSM beams are generated. When the output power of the signal light is constant, the mixing efficiency is measured according to the output amplitude of the intermediate frequency signal. This experiment found that when the signal light is in a linearly polarized state and the local oscillator light is in a linearly polarized state, a circularly polarized state, or an elliptically polarized state, the amplitude of the intermediate frequency signal is 369.6 mv, 146.6 mv, or 92.1 mv, respectively. When the signal light is in a circularly polarized state, the amplitude of the intermediate frequency signal is 446.4 mv, 504.0 mv, or 159.2 mv, respectively. When the signal light is in an elliptical polarization state, the amplitude of the intermediate frequency signal is 94.4 mv, 124.0 mv, or 254.8 mv, respectively.