ing at the problem that the maximum measurement speed of conventional laser coherent detection of high-speed moving target is limited, in this paper, a Doppler measurement method with large dynamic range of four-light coherent mixing is proposed. Compared with traditional laser coherent detection, this method can double the maximum measurement speed and realize the speed measurement of high-speed and ultra-high-speed targets. Since the four-light coherent mixing detection system has one more local oscillator light than the heterodyne laser coherent detection system, the local shot noise increases. Therefore, according to the definition of signal-to-noise ratio, the signal-to-noise ratio formula of four-light coherent mixing is derived. By using the formula, theoretical and numerical analyses are carried out from three aspects of coherence length, receiving aperture, and mismatch angle. The optimal relationship between coherent length and receiving aperture is obtained.