To study the effect of local oscillator power, detector characteristics and polarization on signal-to-noise ratio(SNR) of the space coherent optical communication, the SNR were studied under different working conditions. The effect of characteristic parameters of the detector under different work status and the local oscillator power on the SNR was discussed. The effects on SNR was verified through the experiment. Results show that the characteristic parameters of the detector limit the value of the local oscillator power. Actually, the maximum SNR is less than the theoretical value in coherent optical communication, the optimum value of local oscillator power is greater than the theoretical value, and the closer the two polarization direction of the light beam, the greater the output SNR, the smaller the optimum value of local oscillator power.