Aimming at detecting the micro-Doppler effect of target away, the mathetical model of variational Local Oscillate (LO) power influence to signal to noise ratio(SNR) of balanced heterodyne detection for coherent lidar were established. Besides, simulation analysis on this model was proceeded. The effect on feature extraction was observed through experiments. The results show that in balanced heterodyne detection, where quantum efficiency of two photo electric detectors can′t match completely in reality, there will be a specific LO power making the SNR best, considering the shot noise,hot noise and LO intensity noise. While choosing the optimal LO power in detection, the micro-doppler feature extracted will be more readable and accurate.